sudheerj/javascript-interview-questions

console.log(message); // Output: undefined
var message = "The variable has been hoisted";

var message;
console.log(message); // undefined
message = "The variable has been hoisted";

message("Good morning"); // Output: Good morning

function message(name) {
  console.log(name);
}

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In ES6, JavaScript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance.
For example, the prototype based inheritance written in function expression as below,

```javascript
function Bike(model, color) {
  this.model = model;
  this.color = color;
}

Bike.prototype.getDetails = function () {
  return this.model + " bike has" + this.color + " color";
};
```

Whereas ES6 classes can be defined as an alternative

```javascript
class Bike {
  constructor(color, model) {
    this.color = color;
    this.model = model;
  }

  getDetails() {
    return this.model + " bike has" + this.color + " color";
  }
}
```

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A closure is the combination of a function bundled(enclosed) together with its lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables, functions and other data even after the outer function has finished its execution. The closure has three scope chains.

1. Own scope where variables defined between its curly brackets
2. Outer function's variables
3. Global variables

Let's take an example of closure concept,

```javascript
function Welcome(name) {
  var greetingInfo = function (message) {
    console.log(message + " " + name);
  };
  return greetingInfo;
}
var myFunction = Welcome("John");
myFunction("Welcome "); //Output: Welcome John
myFunction("Hello Mr."); //output: Hello Mr. John
```

As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.

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Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor

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Below are the list of benefits using modules in javascript ecosystem

1. Maintainability
2. Reusability
3. Namespacing

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Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.

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A Service worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences(offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.

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Service worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the `postMessage` interface, and those pages can manipulate the DOM.

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The problem with service worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a service worker's `onfetch` and `onmessage` handlers. In this case, service workers will have access to IndexedDB API in order to persist and reuse across restarts.

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IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.

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Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.

1. **Local storage:** It stores data for current origin with no expiration date.
2. **Session storage:** It stores data for one session and the data is lost when the browser tab is closed.

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Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).

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A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs.
For example, you can create a cookie named username as below,

```javascript
document.cookie = "username=John";
```

![Screenshot](images/cookie.png)

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Cookies are used to remember information about the user profile(such as username). It basically involves two steps,

1. When a user visits a web page, the user profile can be stored in a cookie.
2. Next time the user visits the page, the cookie remembers the user profile.

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There are few below options available for a cookie,

1. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time).

```javascript
document.cookie = "username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC";
```

2. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter.

```javascript
document.cookie = "username=John; path=/services";
```

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You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case.
For example, you can delete a username cookie in the current page as below.

```javascript
document.cookie =
  "username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";
```

**Note:** You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.

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Below are some of the differences between cookie, local storage and session storage,

| Feature                           | Cookie                                                                                                              | Local storage         | Session storage     |
| --------------------------------- | ------------------------------------------------------------------------------------------------------------------- | --------------------- | ------------------- |
| Accessed on client or server side | Both server-side & client-side. The `set-cookie` HTTP response header is used by server inorder to send it to user. | client-side only      | client-side only    |
| Expiry                            | Manually configured using Expires option                                                                            | Forever until deleted | until tab is closed |
| SSL support                       | Supported                                                                                                           | Not supported         | Not supported       |
| Maximum data size                 | 4KB                                                                                                                 | 5 MB                  | 5MB                 |
| Accessible from                   | Any window                                                                                                          | Any window            | Same tab            |
| Sent with requests                | Yes                                                                                                                 | No                    | No                  |

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LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.

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The Window object implements the `WindowLocalStorage` and `WindowSessionStorage` objects which has `localStorage`(window.localStorage) and `sessionStorage`(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local).
For example, you can read and write on local storage objects as below

```javascript
localStorage.setItem("logo", document.getElementById("logo").value);
localStorage.getItem("logo");
```

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The session storage provided methods for reading, writing and clearing the session data

```javascript
// Save data to sessionStorage
sessionStorage.setItem("key", "value");

// Get saved data from sessionStorage
let data = sessionStorage.getItem("key");

// Remove saved data from sessionStorage
sessionStorage.removeItem("key");

// Remove all saved data from sessionStorage
sessionStorage.clear();
```

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The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events.
The syntax would be as below

```javascript
window.onstorage = functionRef;
```

Let's take the example usage of onstorage event handler which logs the storage key and it's values

```javascript
window.onstorage = function (e) {
  console.log(
    "The " +
      e.key +
      " key has been changed from " +
      e.oldValue +
      " to " +
      e.newValue +
      "."
  );
};
```

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Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.

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You need to check browser support for localStorage and sessionStorage before using web storage,

```javascript
if (typeof Storage !== "undefined") {
  // Code for localStorage/sessionStorage.
} else {
  // Sorry! No Web Storage support..
}
```

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You need to check browser support for web workers before using it

```javascript
if (typeof Worker !== "undefined") {
  // code for Web worker support.
} else {
  // Sorry! No Web Worker support..
}
```

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You need to follow below steps to start using web workers for counting example

1. Create a Web Worker File: You need to write a script to increment the count value. Let's name it as counter.js

```javascript
let i = 0;

function timedCount() {
  i = i + 1;
  postMessage(i);
  setTimeout("timedCount()", 500);
}

timedCount();
```

Here postMessage() method is used to post a message back to the HTML page

2. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let's name this file as web_worker_example.js

```javascript
if (typeof w == "undefined") {
  w = new Worker("counter.js");
}
```

and we can receive messages from web worker

```javascript
w.onmessage = function (event) {
  document.getElementById("message").innerHTML = event.data;
};
```

3. Terminate a Web Worker:
   Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.

```javascript
w.terminate();
```

4. Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code

```javascript
w = undefined;
```

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WebWorkers don't have access to below javascript objects since they are defined in an external files

1. Window object
2. Document object
3. Parent object

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A Promise can be in one of **three states**:
- `pending`: Initial state, neither fulfilled nor rejected.
- `fulfilled`: The operation completed successfully.
- `rejected`: The operation failed (e.g., due to a network error).


#### Promise Syntax

```javascript
const promise = new Promise(function (resolve, reject) {
  // Perform async operation
});
```
#### Example: Creating and Using a Promise
```javascript
const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("I'm a Promise!");
  }, 5000);
});

promise
  .then((value) => console.log(value)) // Logs after 5 seconds: "I'm a Promise!"
  .catch((error) => console.error(error))  // Handles any rejection
  .finally(() => console.log("Done"));     // Runs regardless of success or failure
```
In the above example:

*   A `Promise` is created to handle an asynchronous operation with `resolve` and `reject` callbacks.
*   The `setTimeout` resolves the promise with a value after 5 seconds.
*   `.then()`, `.catch()`, and `.finally()` are used to handle success, errors, and cleanup respectively.

The action flow of a promise will be as below,

![Screenshot](images/promises.png)

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Promises are used to:

1.  **Handle asynchronous operations**.
2.  **Provide a cleaner alternative to callbacks**.
3.  **Avoid callback hell**.
4.  **Make code more readable and maintainable**.

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Promises have three states:

1. **Pending:** This is an initial state of the Promise before an operation begins
2. **Fulfilled:** This state indicates that the specified operation was completed.
3. **Rejected:** This state indicates that the operation did not complete. In this case an error value will be thrown.

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A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action.
Let's take a simple example of how to use callback function

```javascript
function callbackFunction(name) {
  console.log("Hello " + name);
}

function outerFunction(callback) {
  let name = prompt("Please enter your name.");
  callback(name);
}

outerFunction(callbackFunction);
```

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The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response, javascript will keep executing while listening for other events.
Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.

```javascript
function firstFunction() {
  // Simulate a code delay
  setTimeout(function () {
    console.log("First function called");
  }, 1000);
}
function secondFunction() {
  console.log("Second function called");
}
firstFunction();
secondFunction();

// Output:
// Second function called
// First function called
```

As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.

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Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,

```javascript
async1(function(){
    async2(function(){
        async3(function(){
            async4(function(){
                ....
            });
        });
    });
});
```

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Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter/X updates, stock price updates, news feeds etc.

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The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,

```javascript
if (typeof EventSource !== "undefined") {
  var source = new EventSource("sse_generator.js");
  source.onmessage = function (event) {
    document.getElementById("output").innerHTML += event.data + "<br>";
  };
}
```

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You can perform browser support for server-sent events before using it as below,

```javascript
if (typeof EventSource !== "undefined") {
  // Server-sent events supported. Let's have some code here!
} else {
  // No server-sent events supported
}
```

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Below are the list of events available for server sent events
| Event | Description |
|---- | ---------
| onopen | It is used when a connection to the server is opened |
| onmessage | This event is used when a message is received |
| onerror | It happens when an error occurs|

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A promise must follow a specific set of rules:

1. A promise is an object that supplies a standard-compliant `.then()` method
2. A pending promise may transition into either fulfilled or rejected state
3. A fulfilled or rejected promise is settled and it must not transition into any other state.
4. Once a promise is settled, the value must not change.

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You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks. Beware, too many levels of nesting lead to [Callback hell](https://github.com/sudheerj/javascript-interview-questions?tab=readme-ov-file#what-is-a-callback-hell)

```javascript
loadScript("/script1.js", function (script) {
  console.log("first script is loaded");

  loadScript("/script2.js", function (script) {
    console.log("second script is loaded");

    loadScript("/script3.js", function (script) {
      console.log("third script is loaded");
      // after all scripts are loaded
    });
  });
});
```

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The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,

```javascript
new Promise(function (resolve, reject) {
  setTimeout(() => resolve(1), 1000);
})
  .then(function (result) {
    console.log(result); // 1
    return result * 2;
  })
  .then(function (result) {
    console.log(result); // 2
    return result * 3;
  })
  .then(function (result) {
    console.log(result); // 6
    return result * 4;
  });
```

In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,

1. The initial promise resolves in 1 second,
2. After that `.then` handler is called by logging the result(1) and then return a promise with the value of result \* 2.
3. After that the value passed to the next `.then` handler by logging the result(2) and return a promise with result \* 3.
4. Finally the value passed to the last `.then` handler by logging the result(6) and return a promise with result \* 4.

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Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,

```javascript
Promise.all([Promise1, Promise2, Promise3]) .then(result) => {   console.log(result) }) .catch(error => console.log(`Error in promises ${error}`))
```

**Note:** Remember that the order of the promises(output the result) is maintained as per input order.

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Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first

```javascript
var promise1 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 500, "one");
});
var promise2 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 100, "two");
});

Promise.race([promise1, promise2]).then(function (value) {
  console.log(value); // "two" // Both promises will resolve, but promise2 is faster
});
```

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JavaScript’s "use strict" directive is used to opt into a stricter parsing and error-handling mode for your scripts or functions. It helps catch common bugs, makes your code more secure, and prepares it for future versions of JavaScript.

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression `"use strict";` instructs the browser to use the javascript code in the Strict mode. This also enables block-scoped variables.

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Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.

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The strict mode is declared by adding "use strict"; to the beginning of a script or a function.
If declared at the beginning of a script, it has global scope.

```javascript
"use strict";
x = 3.14; // This will cause an error because x is not declared
```

and if you declare inside a function, it has local scope

```javascript
x = 3.14; // This will not cause an error.
myFunction();

function myFunction() {
  "use strict";
  y = 3.14; // This will cause an error
}
```

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The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, it will be true.
For example, you can test IE version using this expression as below,

```javascript
let isIE8 = false;
isIE8 = !!navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // returns true or false
```

If you don't use this expression then it returns the original value.

```javascript
console.log(navigator.userAgent.match(/MSIE 8.0/)); // returns either an Array or null
```

**Note:** The expression !! is not an operator, but it is just twice of ! operator.

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The delete operator is used to delete the property as well as its value.

```javascript
var user = { firstName: "John", lastName: "Doe", age: 20 };
delete user.age;

console.log(user); // {firstName: "John", lastName:"Doe"}
```

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You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.

```javascript
typeof "John Abraham"; // Returns "string"
typeof (1 + 2); // Returns "number"
typeof [1, 2, 3]; // Returns "object" because all arrays are also objects
```

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The undefined property indicates that a variable has not been assigned a value, or declared but not initialized at all. The type of undefined value is undefined too.

```javascript
var user; // Value is undefined, type is undefined
console.log(typeof user); //undefined
```

Any variable can be emptied by setting the value to undefined.

```javascript
user = undefined;
```

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The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object.
You can empty the variable by setting the value to null.

```javascript
var user = null;
console.log(typeof user); //object
```

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Below are the main differences between null and undefined,

| Null                                                                                            | Undefined                                                                                               |
| ----------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
| It is an assignment value which indicates that variable points to no object.                    | It is not an assignment value where a variable has been declared but has not yet been assigned a value. |
| Type of null is object                                                                          | Type of undefined is undefined                                                                          |
| The null value is a primitive value that represents the null, empty, or non-existent reference. | The undefined value is a primitive value used when a variable has not been assigned a value.            |
| Indicates the absence of a value for a variable                                                 | Indicates absence of variable itself                                                                    |
| Converted to zero (0) while performing primitive operations                                     | Converted to NaN while performing primitive operations                                                  |

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The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.

```javascript
console.log(eval("1 + 2")); //  3
```

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Below are the main differences between window and document,

| Window                                                                        | Document                                                                                       |
| ----------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------- |
| It is the root level element in any web page                                  | It is the direct child of the window object. This is also known as Document Object Model (DOM) |
| By default window object is available implicitly in the page                  | You can access it via window.document or document.                                             |
| It has methods like alert(), confirm() and properties like document, location | It provides methods like getElementById, getElementsByTagName, createElement etc               |

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The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.

```javascript
function goBack() {
  window.history.back();
}
function goForward() {
  window.history.forward();
}
```

**Note:** You can also access history without window prefix.

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The `mouseEvent getModifierState()` is used to return a boolean value that indicates whether the specified modifier key is activated or not. The modifiers such as CapsLock, ScrollLock and NumLock are activated when they are clicked, and deactivated when they are clicked again.

Let's take an input element to detect the CapsLock on/off behavior with an example:

```html
<input type="password" onmousedown="enterInput(event)" />

<p id="feedback"></p>

<script>
  function enterInput(e) {
    var flag = e.getModifierState("CapsLock");
    if (flag) {
      document.getElementById("feedback").innerHTML = "CapsLock activated";
    } else {
      document.getElementById("feedback").innerHTML =
        "CapsLock not activated";
    }
  }
</script>
```

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The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.

```javascript
isNaN("Hello"); //true
isNaN("100"); //false
```

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Below are the major differences between undeclared(not defined) and undefined variables,

| undeclared                                                                                  | undefined                                                                              |
| ------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------- |
| These variables do not exist in a program and are not declared                              | These variables declared in the program but have not assigned any value                |
| If you try to read the value of an undeclared variable, then a runtime error is encountered | If you try to read the value of an undefined variable, an undefined value is returned. |

```javascript
var a;
a; // yields undefined

b; // Throws runtime error like "Uncaught ReferenceError: b is not defined"
```
This can be confusing, because it says `not defined` instead of `not declared` (Chrome)

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Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable

```javascript
msg = "Hello"; // var is missing, it becomes global variable
```

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The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.

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The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases

```javascript
Math.sqrt(-1);
parseInt("Hello");
```

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The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.

```javascript
isFinite(Infinity); // false
isFinite(NaN); // false
isFinite(-Infinity); // false

isFinite(100); // true
```

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Event flow refers to the order in which events are handled in the browser when a user interacts with elements on a webpage like clicking, typing, hovering, etc.

When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object.

Hence, there are three phases in JavaScript’s event flow:

1. Event Capturing(Top to Bottom): The event starts from the window/document and moves down the DOM tree toward the target element.
2. Target phase: The event reaches the target element — the element that was actually interacted with.
3. Event Bubbling(Bottom to Top): The event then bubbles back up from the target element to the root.

**[⬆ Back to Top](#table-of-contents)**

Event capturing is a phase of event propagation in which an event is first intercepted by the outermost ancestor element, then travels downward through the DOM hierarchy until it reaches the target (innermost) element.

To handle events during the capturing phase, you need to pass `true` as the third argument to the `addEventListener` method.

 ```javascript
  <div>
    <button class="child">Hello</button>
  </div>

  <script>
    const parent = document.querySelector("div");
    const child = document.querySelector(".child");

    // Capturing phase: parent listener (runs first)
    parent.addEventListener("click", function () {
      console.log("Parent (capturing)");
    }, true); // `true` enables capturing

    // Bubbling phase: child listener (runs after)
    child.addEventListener("click", function () {
      console.log("Child (bubbling)");
    });
  </script>
  // Parent (capturing)
  // Child (bubbling)
 ```

**[⬆ Back to Top](#table-of-contents)**

Event bubbling is a type of event propagation in which an event first triggers on the innermost target element (the one the user interacted with), and then bubbles up through its ancestors in the DOM hierarchy — eventually reaching the outermost elements, like the document or window.

By default, event listeners in JavaScript are triggered during the bubbling phase, unless specified otherwise.

```javascript
<div>
  <button class="child">Hello</button>
</div>

<script>
  const parent = document.querySelector("div");
  const child = document.querySelector(".child");

  // Bubbling phase (default)
  parent.addEventListener("click", function () {
    console.log("Parent");
  });

  child.addEventListener("click", function () {
    console.log("Child");
  });
</script>
//Child
//Parent
```

Here, at first, the event triggers on the child button. Thereafter it bubbles up and triggers the parent div's event handler.

**[⬆ Back to Top](#table-of-contents)**

You can submit a form using `document.forms[0].submit()`. All the form input's information is submitted using onsubmit event handler

```javascript
function submit() {
  document.forms[0].submit();
}
```

**[⬆ Back to Top](#table-of-contents)**

The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,

```javascript
console.log(navigator.platform);
```

**[⬆ Back to Top](#table-of-contents)**

The `DOMContentLoaded` event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).

**[⬆ Back to Top](#table-of-contents)**

`Native objects` are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec.
`Host objects` are objects provided by the browser or runtime environment (Node).

For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects.
`User objects` are objects defined in the javascript code. For example, User objects created for profile information.

**[⬆ Back to Top](#table-of-contents)**

You can use below tools or techniques for debugging javascript

1. Chrome Devtools
2. debugger statement
3. Good old console.log statement

**[⬆ Back to Top](#table-of-contents)**

Below are the list of pros and cons of promises over callbacks,

**Pros:**

1. It avoids callback hell which is unreadable
2. Easy to write sequential asynchronous code with .then()
3. Easy to write parallel asynchronous code with Promise.all()
4. Solves some of the common problems of callbacks(call the callback too late, too early, many times and swallow errors/exceptions)

**Cons:**

5. It makes little complex code
6. You need to load a polyfill if ES6 is not supported

**[⬆ Back to Top](#table-of-contents)**

Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes: `type` and `value`,

```javascript
<input type="text" value="Name:">
```

You can retrieve the attribute value as below, for example after typing "Good morning" into the input field:

```javascript
const input = document.querySelector("input");
console.log(input.getAttribute("value")); // Good morning
console.log(input.value); // Good morning
```

And after you change the value of the text field to "Good evening", it becomes like

```javascript
console.log(input.getAttribute("value")); // Good evening
console.log(input.value); // Good evening
```

**[⬆ Back to Top](#table-of-contents)**

The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).

**[⬆ Back to Top](#table-of-contents)**

Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an `<a>` element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression.
For example, the below link notify the message without reloading the page

```javascript
<a href="JavaScript:void(0);" onclick="alert('Well done!')">
  Click Me!
</a>
```

**[⬆ Back to Top](#table-of-contents)**

JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.

**[⬆ Back to Top](#table-of-contents)**

Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.

**[⬆ Back to Top](#table-of-contents)**

  No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).

  **[⬆ Back to Top](#table-of-contents)**

  Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can `react` on these events. Some of the examples of HTML events are,

  1. Web page has finished loading
  2. Input field was changed
  3. Button was clicked

  Let's describe the behavior of click event for button element,

  ```javascript
  <!doctype html>
  <html>
   <head>
     <script>
       function greeting() {
         alert('Hello! Good morning');
       }
     </script>
   </head>
   <body>
     <button type="button" onclick="greeting()">Click me</button>
   </body>
  </html>
  ```

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name `Mocha`, but later the language was officially called `LiveScript` when it first shipped in beta releases of Netscape.

  **[⬆ Back to Top](#table-of-contents)**

  The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.

  ```javascript
  document
    .getElementById("link")
    .addEventListener("click", function (event) {
      event.preventDefault();
    });
  ```

  **Note:** Remember that not all events are cancelable.

  **[⬆ Back to Top](#table-of-contents)**

  The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)

  ```javascript
  <p>Click DIV1 Element</p>
  <div onclick="secondFunc()">DIV 2
    <div onclick="firstFunc(event)">DIV 1</div>
  </div>

  <script>
  function firstFunc(event) {
    alert("DIV 1");
    event.stopPropagation();
  }

  function secondFunc() {
    alert("DIV 2");
  }
  </script>
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The return false statement in event handlers performs the below steps,

  1. First it stops the browser's default action or behaviour.
  2. It prevents the event from propagating the DOM
  3. Stops callback execution and returns immediately when called.

  **[⬆ Back to Top](#table-of-contents)**

  The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.

  ![Screenshot](images/bom.png)

  **[⬆ Back to Top](#table-of-contents)**

  The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method,

  ```javascript
  setTimeout(function () {
    console.log("Good morning");
  }, 2000);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method,

  ```javascript
  setInterval(function () {
    console.log("Good morning");
  }, 2000);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.

  **[⬆ Back to Top](#table-of-contents)**

  Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.

  For example, if you wanted to detect field changes inside a specific form, you can use event delegation technique,

  ```javascript
  var form = document.querySelector("#registration-form");

  // Listen for changes to fields inside the form
  form.addEventListener(
    "input",
    function (event) {
      // Log the field that was changed
      console.log(event.target);
    },
    false
  );
  ```

  **[⬆ Back to Top](#table-of-contents)**

  ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.

  **[⬆ Back to Top](#table-of-contents)**

  JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of syntax rules of JSON

  1. The data is in name/value pairs
  2. The data is separated by commas
  3. Curly braces hold objects
  4. Square brackets hold arrays

  **[⬆ Back to Top](#table-of-contents)**

  When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.

  ```javascript
  var userJSON = { name: "John", age: 31 };
  var userString = JSON.stringify(userJSON);
  console.log(userString); //"{"name":"John","age":31}"
  ```

  **[⬆ Back to Top](#table-of-contents)**

  When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.

  ```javascript
  var userString = '{"name":"John","age":31}';
  var userJSON = JSON.parse(userString);
  console.log(userJSON); // {name: "John", age: 31}
  ```

  **[⬆ Back to Top](#table-of-contents)**

  When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.

  **[⬆ Back to Top](#table-of-contents)**

  Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.

  **[⬆ Back to Top](#table-of-contents)**

  The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.

  For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.

  ```javascript
  <script>
       var msg;
       function greeting() {
          alert('Good morning');
       }
       function start() {
         msg =setTimeout(greeting, 3000);

       }

       function stop() {
           clearTimeout(msg);
       }
  </script>
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.

  For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.

  ```javascript
  <script>
       var msg;
       function greeting() {
          alert('Good morning');
       }
       function start() {
         msg = setInterval(greeting, 3000);

       }

       function stop() {
           clearInterval(msg);
       }
  </script>
  ```

  **[⬆ Back to Top](#table-of-contents)**

  In vanilla javascript, you can redirect to a new page using the `location` property of window object. The syntax would be as follows,

  ```javascript
  function redirect() {
    window.location.href = "newPage.html";
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  There are 3 possible ways to check whether a string contains a substring or not,

  1. **Using includes:** ES6 provided `String.prototype.includes` method to test a string contains a substring

  ```javascript
  var mainString = "hello",
    subString = "hell";
  mainString.includes(subString);
  ```

  2. **Using indexOf:** In an ES5 or older environment, you can use `String.prototype.indexOf` which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.

  ```javascript
  var mainString = "hello",
    subString = "hell";
  mainString.indexOf(subString) !== -1;
  ```

  3. **Using RegEx:** The advanced solution is using Regular expression's test method(`RegExp.test`), which allows for testing for against regular expressions

  ```javascript
  var mainString = "hello",
    regex = /hell/;
  regex.test(mainString);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.

  ```javascript
  function validateEmail(email) {
    var re =
      /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    return re.test(String(email).toLowerCase());
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The above regular expression accepts unicode characters.

  You can use `window.location.href` expression to get the current url path and you can use the same expression for updating the URL too. You can also use `document.URL` for read-only purposes but this solution has issues in FF.

  ```javascript
  console.log("location.href", window.location.href); // Returns full URL
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The below `Location` object properties can be used to access URL components of the page,

  1. href - The entire URL
  2. protocol - The protocol of the URL
  3. host - The hostname and port of the URL
  4. hostname - The hostname of the URL
  5. port - The port number in the URL
  6. pathname - The path name of the URL
  7. search - The query portion of the URL
  8. hash - The anchor portion of the URL

  **[⬆ Back to Top](#table-of-contents)**

  You can use URLSearchParams to get query string values in javascript. Let's see an example to get the client code value from URL query string,

  ```javascript
  const urlParams = new URLSearchParams(window.location.search);
  const clientCode = urlParams.get("clientCode");
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can check whether a key exists in an object or not using three approaches,

  1. **Using in operator:** You can use the in operator whether a key exists in an object or not

     ```javascript
     "key" in obj;
     ```

     and If you want to check if a key doesn't exist, remember to use parenthesis,

     ```javascript
     !("key" in obj);
     ```

  2. **Using hasOwnProperty method:** You can use `hasOwnProperty` to particularly test for properties of the object instance (and not inherited properties)

     ```javascript
     obj.hasOwnProperty("key"); // true
     ```

  3. **Using undefined comparison:** If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property.

     ```javascript
     const user = {
       name: "John",
     };

     console.log(user.name !== undefined); // true
     console.log(user.nickName !== undefined); // false
     ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `for-in` loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype using `hasOwnProperty` method.

  ```javascript
  var object = {
    k1: "value1",
    k2: "value2",
    k3: "value3",
  };

  for (var key in object) {
    if (object.hasOwnProperty(key)) {
      console.log(key + " -> " + object[key]); // k1 -> value1 ...
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  There are different solutions based on ECMAScript versions

  1. **Using Object entries(ECMA 7+):** You can use object entries length along with constructor type.

  ```javascript
  Object.entries(obj).length === 0 && obj.constructor === Object; // Since date object length is 0, you need to check constructor check as well
  ```

  2. **Using Object keys(ECMA 5+):** You can use object keys length along with constructor type.

  ```javascript
  Object.keys(obj).length === 0 && obj.constructor === Object; // Since date object length is 0, you need to check constructor check as well
  ```

  3. **Using for-in with hasOwnProperty(Pre-ECMA 5):** You can use a for-in loop along with hasOwnProperty.

  ```javascript
  function isEmpty(obj) {
    for (var prop in obj) {
      if (obj.hasOwnProperty(prop)) {
        return false;
      }
    }

    return JSON.stringify(obj) === JSON.stringify({});
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function,

  ```javascript
  function sum() {
    var total = 0;
    for (var i = 0, len = arguments.length; i < len; ++i) {
      total += arguments[i];
    }
    return total;
  }

  sum(1, 2, 3); // returns 6
  ```

  **Note:** You can't apply array methods on arguments object. But you can convert into a regular array as below.

  ```javascript
  var argsArray = Array.prototype.slice.call(arguments);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.

  ```javascript
  function capitalizeFirstLetter(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The for-loop is a commonly used iteration syntax in javascript. It has both pros and cons

  #### Pros

  1. Works on every environment
  2. You can use break and continue flow control statements

  #### Cons

  3. Too verbose
  4. Imperative
  5. You might face off-by-one errors.

  **[⬆ Back to Top](#table-of-contents)**

  You can use `new Date()` to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyy

  ```javascript
  var today = new Date();
  var dd = String(today.getDate()).padStart(2, "0");
  var mm = String(today.getMonth() + 1).padStart(2, "0"); //January is 0!
  var yyyy = today.getFullYear();

  today = mm + "/" + dd + "/" + yyyy;
  document.write(today);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You need to use date.getTime() method in order to compare unix timestamp values

  ```javascript
  var d1 = new Date();
  var d2 = new Date(d1);
  console.log(d1.getTime() === d2.getTime()); //True
  console.log(d1 === d2); // False
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use ECMAScript 6's `String.prototype.startsWith()` method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage,

  ```javascript
  "Good morning".startsWith("Good"); // true
  "Good morning".startsWith("morning"); // false
  ```

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.

  ```javascript
  "  Hello World   ".trim(); //Hello World
  ```

  If your browser(<IE9) doesn't support this method then you can use below polyfill.

  ```javascript
  if (!String.prototype.trim) {
    (function () {
      // Make sure we trim BOM and NBSP
      var rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
      String.prototype.trim = function () {
        return this.replace(rtrim, "");
      };
    })();
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  There are two possible solutions to add new properties to an object.

  Let's take a simple object to explain these solutions.

  ```javascript
  var object = {
    key1: value1,
    key2: value2,
  };
  ```

  1. **Using dot notation:** This solution is useful when you know the name of the property

  ```javascript
  object.key3 = "value3";
  ```

  2. **Using square bracket notation:** This solution is useful when the name of the property is dynamically determined or the key's name is non-JS like "user-name"

  ```javascript
  obj["key3"] = "value3";
  ```

  **[⬆ Back to Top](#table-of-contents)**

  No,that's not a special operator. But it is a combination of 2 standard operators one after the other,

  1. A logical not (!)
  2. A prefix decrement (--)

  At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.

  **[⬆ Back to Top](#table-of-contents)**

  You can use the logical or operator `||` in an assignment expression to provide a default value. The syntax looks like as below,

  ```javascript
  var a = b || c;
  ```

  As per the above expression, variable 'a 'will get the value of 'c' only if 'b' is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise 'a' will get the value of 'b'.

  **[⬆ Back to Top](#table-of-contents)**

  You can define multiline string literals using the '\n' character followed by line terminator('\').

  ```javascript
  var str = "This is a \n very lengthy \n sentence!";
  console.log(str);
  ```

  But if you have a space after the '\n' character, there will be indentation inconsistencies.

  **[⬆ Back to Top](#table-of-contents)**

  An application shell (or app shell) architecture is one way to build a Progressive Web App that reliably and instantly loads on your users' screens, similar to what you see in native applications. It is useful for getting some initial HTML to the screen fast without a network.

  **[⬆ Back to Top](#table-of-contents)**

  Yes, we can define properties for functions because functions are also objects.

  ```javascript
  fn = function (x) {
    //Function code goes here
  };

  fn.name = "John";

  fn.profile = function (y) {
    //Profile code goes here
  };
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use `function.length` syntax to find the number of parameters expected by a function. Let's take an example of `sum` function to calculate the sum of numbers,

  ```javascript
  function sum(num1, num2, num3, num4) {
    return num1 + num2 + num3 + num4;
  }
  sum.length; // 4 is the number of parameters expected.
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.

  There are two main polyfill libraries available,

  1. **Core.js**: It is a modular javascript library used for cutting-edge ECMAScript features.
  2. **Polyfill.io:** It provides polyfills that are required for browser needs.

  **[⬆ Back to Top](#table-of-contents)**

  The break statement is used to "jump out" of a loop. i.e, It breaks the loop and continues executing the code after the loop.

  ```javascript
  for (i = 0; i < 10; i++) {
    if (i === 5) {
      break;
    }
    text += "Number: " + i + "<br>";
  }
  ```

  The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

  ```javascript
  for (i = 0; i < 10; i++) {
    if (i === 5) {
      continue;
    }
    text += "Number: " + i + "<br>";
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,

  ```javascript
  var i, j;

  loop1: for (i = 0; i < 3; i++) {
    loop2: for (j = 0; j < 3; j++) {
      if (i === j) {
        continue loop1;
      }
      console.log("i = " + i + ", j = " + j);
    }
  }

  // Output is:
  //   "i = 1, j = 0"
  //   "i = 2, j = 0"
  //   "i = 2, j = 1"
  ```

  **[⬆ Back to Top](#table-of-contents)**

  It is recommended to keep all declarations at the top of each script or function. The benefits of doing this are,

  1. Gives cleaner code
  2. It provides a single place to look for local variables
  3. Easy to avoid unwanted global variables
  4. It reduces the possibility of unwanted re-declarations

  **[⬆ Back to Top](#table-of-contents)**

  It is recommended to initialize variables because of the below benefits,

  1. It gives cleaner code
  2. It provides a single place to initialize variables
  3. Avoid undefined values in the code

  **[⬆ Back to Top](#table-of-contents)**

  It is recommended to avoid creating new objects using `new Object()`. Instead you can initialize values based on it's type to create the objects.

  1. Assign {} instead of new Object()
  2. Assign "" instead of new String()
  3. Assign 0 instead of new Number()
  4. Assign false instead of new Boolean()
  5. Assign [] instead of new Array()
  6. Assign /()/ instead of new RegExp()
  7. Assign function (){} instead of new Function()

  You can define them as an example,

  ```javascript
  var v1 = {};
  var v2 = "";
  var v3 = 0;
  var v4 = false;
  var v5 = [];
  var v6 = /()/;
  var v7 = function () {};
  ```

  **[⬆ Back to Top](#table-of-contents)**

  JSON arrays are written inside square brackets and arrays contain javascript objects. For example, the JSON array of users would be as below,

  ```javascript
  "users":[
    {"firstName":"John", "lastName":"Abrahm"},
    {"firstName":"Anna", "lastName":"Smith"},
    {"firstName":"Shane", "lastName":"Warn"}
  ]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use `Math.random()` with `Math.floor()` to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,

  ```javascript
  Math.floor(Math.random() * 10) + 1; // returns a random integer from 1 to 10
  Math.floor(Math.random() * 100) + 1; // returns a random integer from 1 to 100
  ```

  **Note:** `Math.random()` returns a random number between 0 (inclusive), and 1 (exclusive)

  **[⬆ Back to Top](#table-of-contents)**

  Yes, you can create a proper random function to return a random number between min and max (both included)

  ```javascript
  function randomInteger(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
  }
  randomInteger(1, 100); // returns a random integer from 1 to 100
  randomInteger(1, 1000); // returns a random integer from 1 to 1000
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler `rollup`, these days practically all bundlers use this technique.

  **[⬆ Back to Top](#table-of-contents)**

  Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.

  **[⬆ Back to Top](#table-of-contents)**

  No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.

  **[⬆ Back to Top](#table-of-contents)**

  A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let's see the syntax format now,

  ```javascript
  /pattern/modifiers;
  ```

  For example, the regular expression or search pattern with case-insensitive username would be,

  ```javascript
  /John/i;
  ```

  **[⬆ Back to Top](#table-of-contents)**

  There are six string methods: `search()`, `replace()`, `replaceAll()`, `match()`, `matchAll()`, and `split()`.

  The `search()` method uses an expression to search for a match, and returns the position of the match.

  ```javascript
  var msg = "Hello John";
  var n = msg.search(/John/i); // 6
  ```

  The `replace()` and `replaceAll()` methods are used to return a modified string where the pattern is replaced.

  ```javascript
  var msg = "ball bat";
  var n1 = msg.replace(/b/i, "c"); // call bat
  var n2 = msg.replaceAll(/b/i, "c"); // call cat
  ```

  The `match()` and `matchAll()` methods are used to return the matches when matching a string against a regular expression.

  ```javascript
  var msg = "Hello John";
  var n1 = msg.match(/[A-Z]/g); // ["H", "J"]
  var n2 = msg.matchAll(/[A-Z]/g); // this returns an iterator
  ```

  The `split()` method is used to split a string into an array of substrings, and returns the new array.

  ```javascript
  var msg = "Hello John";
  var n = msg.split(/\s/); // ["Hello", "John"]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Modifiers can be used to perform case-insensitive and global searches. Let's list some of the modifiers,

  | Modifier | Description                                             |
  | -------- | ------------------------------------------------------- |
  | i        | Perform case-insensitive matching                       |
  | g        | Perform a global match rather than stops at first match |
  | m        | Perform multiline matching                              |

  Let's take an example of global modifier,

  ```javascript
  var text = "Learn JS one by one";
  var pattern = /one/g;
  var result = text.match(pattern); // one,one
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types,

  1. **Brackets:** These are used to find a range of characters.
     For example, below are some use cases,
     1. [abc]: Used to find any of the characters between the brackets(a,b,c)
     2. [0-9]: Used to find any of the digits between the brackets
     3. (a|b): Used to find any of the alternatives separated with |
  2. **Metacharacters:** These are characters with a special meaning.
     For example, below are some use cases,
     1. \\d: Used to find a digit
     2. \\s: Used to find a whitespace character
     3. \\b: Used to find a match at the beginning or ending of a word
  3. **Quantifiers:** These are useful to define quantities.
     For example, below are some use cases,
     1. n+: Used to find matches for any string that contains at least one n
     2. n\*: Used to find matches for any string that contains zero or more occurrences of n
     3. n?: Used to find matches for any string that contains zero or one occurrences of n

  **[⬆ Back to Top](#table-of-contents)**

  RegExp object is a regular expression object with predefined properties and methods. Let's see the simple usage of RegExp object,

  ```javascript
  var regexp = new RegExp("\\w+");
  console.log(regexp);
  // expected output: /\w+/
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `test()` method of regular expression in order to search a string for a pattern, and return true or false depending on the result.

  ```javascript
  var pattern = /you/;
  console.log(pattern.test("How are you?")); //true
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.

  ```javascript
  var pattern = /you/;
  console.log(pattern.exec("How are you?")); //["you", index: 8, input: "How are you?", groups: undefined]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can change inline style or classname of a HTML element using javascript DOM-manipulation

  1. **Using style property:** You can modify inline style using style property

  ```javascript
  document.getElementById("title").style.fontSize = "30px";
  ```

  2. **Using ClassName property:** It is easy to modify element class using className property

  ```javascript
  document.getElementById("title").className = "custom-title";
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The output is going to be `33`. Since `1` and `2` are numeric values, the result of the first two digits is going to be a numeric value `3`. The next digit is a string type value because of that the addition of numeric value `3` and string type value `3` is just going to be a concatenation value `33`. Other operations like `3 * '3'` do yield correct results because the string is coerced into a number.

  **[⬆ Back to Top](#table-of-contents)**

  The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect.
  For example, in the below function a debugger statement has been inserted. So
  execution is paused at the debugger statement just like a breakpoint in the script source.

  ```javascript
  function getProfile() {
    // code goes here
    debugger;
    // code goes here
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.

  **[⬆ Back to Top](#table-of-contents)**

  No, you cannot use the reserved words as variables, labels, object or function names. Let's see one simple example,

  ```javascript
  var else = "hello"; // Uncaught SyntaxError: Unexpected token else
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.

  ```javascript
  window.mobilecheck = function () {
    var mobileCheck = false;
    (function (a) {
      if (
        /(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce|xda|xiino/i.test(
          a
        ) ||
        /1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i.test(
          a.substr(0, 4)
        )
      )
        mobileCheck = true;
    })(navigator.userAgent || navigator.vendor || window.opera);
    return mobileCheck;
  };
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage,

  ```javascript
  function detectmob() {
    if (
      navigator.userAgent.match(/Android/i) ||
      navigator.userAgent.match(/webOS/i) ||
      navigator.userAgent.match(/iPhone/i) ||
      navigator.userAgent.match(/iPad/i) ||
      navigator.userAgent.match(/iPod/i) ||
      navigator.userAgent.match(/BlackBerry/i) ||
      navigator.userAgent.match(/Windows Phone/i)
    ) {
      return true;
    } else {
      return false;
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can programmatically get the image and check the dimensions(width and height) using JavaScript.

  ```javascript
  var img = new Image();
  img.onload = function () {
    console.log(this.width + "x" + this.height);
  };
  img.src = "http://www.google.com/intl/en_ALL/images/logo.gif";
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Browsers provide an XMLHttpRequest object which can be used to make synchronous HTTP requests from JavaScript.

  ```javascript
  function httpGet(theUrl) {
    var xmlHttpReq = new XMLHttpRequest();
    xmlHttpReq.open("GET", theUrl, false); // false for synchronous request
    xmlHttpReq.send(null);
    return xmlHttpReq.responseText;
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.

  ```javascript
  function httpGetAsync(theUrl, callback) {
    var xmlHttpReq = new XMLHttpRequest();
    xmlHttpReq.onreadystatechange = function () {
      if (xmlHttpReq.readyState == 4 && xmlHttpReq.status == 200)
        callback(xmlHttpReq.responseText);
    };
    xmlHttpReq.open("GET", theUrl, true); // true for asynchronous
    xmlHttpReq.send(null);
  }
  ```

  Today this is considered deprecated, because an async `fetch` call (in browsers later than 2016) is simpler and more robust.

  **[⬆ Back to Top](#table-of-contents)**

  You can use the toLocaleString() method to convert dates in one timezone to another. For example, let's convert current date to British English timezone as below,

  ```javascript
  console.log(new Date().toLocaleString("en-GB", { timeZone: "UTC" })); //29/06/2019, 09:56:00
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let's use a combination of these properties to calculate the size of a window or document,

  ```javascript
  var width =
    window.innerWidth ||
    document.documentElement.clientWidth ||
    document.body.clientWidth;

  var height =
    window.innerHeight ||
    document.documentElement.clientHeight ||
    document.body.clientHeight;
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The conditional (ternary) operator is the only JavaScript operator that takes three operands which acts as a shortcut for `if` statements.

  ```javascript
  var isAuthenticated = false;
  console.log(
    isAuthenticated ? "Hello, welcome" : "Sorry, you are not authenticated"
  ); // Sorry, you are not authenticated
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Yes, you can apply chaining on conditional operators similar to **`if … else if … else if … else`** chain. The syntax is going to be as below,

  ```javascript
  function traceValue(someParam) {
    return condition1
      ? value1
      : condition2
      ? value2
      : condition3
      ? value3
      : value4;
  }

  // The above conditional operator is equivalent to:

  function traceValue(someParam) {
    if (condition1) {
      return value1;
    } else if (condition2) {
      return value2;
    } else if (condition3) {
      return value3;
    } else {
      return value4;
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can execute javascript after page load in many different ways,

  1. **window.onload:**

  ```javascript
  window.onload = function ...
  ```

  2. **document.onload:**

  ```javascript
  document.onload = function ...
  ```

  3. **body onload:**

  ```javascript
  <body onload="script();">
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `__proto__` object is the actual object that is used in the lookup chain to resolve methods, etc. Whereas `prototype` is the object that is used to build `__proto__` when you create an object with the `new` operator (a special variant of a function call).

  ```javascript
  new Employee().__proto__ === Employee.prototype;
  new Employee().prototype === undefined;
  ```

  There are few more differences,

  | feature    | Prototype                                                    | proto                                                      |
  | ---------- | ------------------------------------------------------------ | ---------------------------------------------------------- |
  | Access     | All function constructors have prototype properties.         | All objects have \_\_proto\_\_ property                    |
  | Purpose    | Used to reduce memory wastage with a single copy of function | Used in lookup chain to resolve methods, constructors etc. |
  | ECMAScript | Introduced in ES6                                            | Introduced in ES5                                          |
  | Usage      | Frequently used                                              | Rarely used                                                |

  **[⬆ Back to Top](#table-of-contents)**

  It is recommended to use semicolons after every statement in JavaScript. For example, in the below case (that is an IIFE = Immediately Invoked Function Expression) it throws an error ".. is not a function" at runtime due to missing semicolon.

  ```javascript
  // define a function
  var fn = (function () {
    //...
  })(
    // semicolon missing at this line

    // then execute some code inside a closure
    function () {
      //...
    }
  )();
  ```

  and it will be interpreted as

  ```javascript
  var fn = (function () {
    //...
  })(function () {
    //...
  })();
  ```

  In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a "... is not a function" error at runtime.

  **[⬆ Back to Top](#table-of-contents)**

  The **`freeze()`** method is used to freeze an object. Freezing an object does not allow adding new properties to an object, prevents removing, and prevents changing the enumerability, configurability, or writability of existing properties. i.e. It returns the passed object and does not create a frozen copy.

  ```javascript
  const obj = {
    prop: 100,
  };

  Object.freeze(obj);
  obj.prop = 200; // Throws an error in strict mode

  console.log(obj.prop); //100
  ```

  Remember freezing is only applied to the top-level properties in objects but not for nested objects.
  For example, let's try to freeze user object which has employment details as nested object and observe that details have been changed.

  ```javascript
  const user = {
    name: "John",
    employment: {
      department: "IT",
    },
  };

  Object.freeze(user);
  user.employment.department = "HR";
  ```

  **Note:** It causes a TypeError if the argument passed is not an object.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the main benefits of using freeze method,

  1. It is used for freezing objects and arrays.
  2. It is used to make an object immutable.

  **[⬆ Back to Top](#table-of-contents)**

  In the Object-oriented paradigm, an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. Hence it works as the `final` keyword which is used in various languages.

  **[⬆ Back to Top](#table-of-contents)**

  You can use the navigator object to detect a browser language preference as below,

  ```javascript
  var language =
    (navigator.languages && navigator.languages[0]) || // Chrome / Firefox
    navigator.language || // All browsers
    navigator.userLanguage; // IE <= 10

  console.log(language);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Title case means that the first letter of each word is capitalized. You can convert a string to title case using the below function,

  ```javascript
  function toTitleCase(str) {
    return str.replace(/\w\S*/g, function (txt) {
      return txt.charAt(0).toUpperCase() + txt.substring(1).toLowerCase();
    });
  }
  toTitleCase("good morning john"); // Good Morning John
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `<noscript>` tag to detect whether JavaScript is disabled or not. The code block inside `<noscript>` gets executed when JavaScript is disabled, and is typically used to display alternative content when the page is generated in JavaScript.

  ```javascript
  <script type="javascript">
      // JS related code goes here
  </script>
  <noscript>
      <a href="next_page.html?noJS=true">JavaScript is disabled in the page. Please click Next Page</a>
  </noscript>
  ```

  **[⬆ Back to Top](#table-of-contents)**

  An operator is capable of manipulating(mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below,

  1. **Arithmetic Operators:** Includes + (Addition), – (Subtraction), \* (Multiplication), / (Division), % (Modulus), ++ (Increment) and – – (Decrement)
  2. **Comparison Operators:** Includes == (Equal), != (Not Equal), === (Equal with type), > (Greater than), >= (Greater than or Equal to), < (Less than), <= (Less than or Equal to)
  3. **Logical Operators:** Includes && (Logical AND), || (Logical OR), ! (Logical NOT)
  4. **Assignment Operators:** Includes = (Assignment Operator), += (Add and Assignment Operator), –= (Subtract and Assignment Operator), \*= (Multiply and Assignment), /= (Divide and Assignment), %= (Modules and Assignment)
  5. **Ternary Operators:** It includes conditional(: ?) Operator
  6. **typeof Operator:** It uses to find type of variable. The syntax looks like `typeof variable`

  **[⬆ Back to Top](#table-of-contents)**

  Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below,

  ```javascript
  function f(a, b, ...theArgs) {
    // ...
  }
  ```

  For example, let's take a sum example to calculate on dynamic number of parameters,

  ```javascript
  function sum(...args) {
    let total = 0;
    for (const i of args) {
      total += i;
    }
    return total;
  }

  console.log(sum(1, 2)); //3
  console.log(sum(1, 2, 3)); //6
  console.log(sum(1, 2, 3, 4)); //10
  console.log(sum(1, 2, 3, 4, 5)); //15
  ```

  **Note:** Rest parameter is added in ES2015 or ES6

  **[⬆ Back to Top](#table-of-contents)**

  The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.

  ```javascript
  function someFunc(a,…b,c){
  //You code goes here
  return;
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of bitwise logical operators used in JavaScript

  1. Bitwise AND ( & )
  2. Bitwise OR ( | )
  3. Bitwise XOR ( ^ )
  4. Bitwise NOT ( ~ )
  5. Left Shift ( << )
  6. Sign Propagating Right Shift ( >> )
  7. Zero fill Right Shift ( >>> )

  **[⬆ Back to Top](#table-of-contents)**

  Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. Let's take an example to see this behavior,

  ```javascript
  function calculateSum(x, y, z) {
    return x + y + z;
  }

  const numbers = [1, 2, 3];

  console.log(calculateSum(...numbers)); // 6
  ```

  **[⬆ Back to Top](#table-of-contents)**

  `Object.isFrozen()` method is used to determine if an object is frozen or not.An object is frozen if all of the below conditions hold true,

  1. If it is not extensible.
  2. If all of its properties are non-configurable.
  3. If all its data properties are non-writable.
     The usage is going to be as follows,

  ```javascript
  const object = {
    property: "Welcome JS world",
  };
  Object.freeze(object);
  console.log(Object.isFrozen(object));
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.is()` method determines whether two values are the same value. For example, the usage with different types of values would be,

  ```javascript
  Object.is("hello", "hello"); // true
  Object.is(window, window); // true
  Object.is([], []); // false
  ```

  Two values are considered identical if one of the following holds:

  1. both undefined
  2. both null
  3. both true or both false
  4. both strings of the same length with the same characters in the same order
  5. both the same object (means both object have same reference)
  6. both numbers and
     both +0
     both -0
     both NaN
     both non-zero and both not NaN and both have the same value.

  **[⬆ Back to Top](#table-of-contents)**

  Some of the applications of Object's `is` method are follows,

  1. It is used for comparison of two strings.
  2. It is used for comparison of two numbers.
  3. It is used for comparing the polarity of two numbers.
  4. It is used for comparison of two objects.

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Object.assign()` method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the source objects. The syntax would be as below,

  ```javascript
  Object.assign(target, ...sources);
  ```

  Let's take example with one source and one target object,

  ```javascript
  const target = { a: 1, b: 2 };
  const source = { b: 3, c: 4 };

  const returnedTarget = Object.assign(target, source);

  console.log(target); // { a: 1, b: 3, c: 4 }

  console.log(returnedTarget); // { a: 1, b: 3, c: 4 }
  ```

  As observed in the above code, there is a common property(`b`) from source to target so it's value has been overwritten.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the some of main applications of `Object.assign()` method,

  1. It is used for cloning an object.
  2. It is used to merge objects with the same properties.

  **[⬆ Back to Top](#table-of-contents)**

  The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc.

  A proxy is created with two parameters: a target object which you want to proxy and a handler object which contains methods to intercept fundamental operations. The syntax would be as follows,

  ```javascript
  var p = new Proxy(target, handler);
  ```

  Let's take a look at below examples of proxy object and how the get method which customize the lookup behavior,

  ```javascript
  //Example1:

  const person = {
    name: "Sudheer Jonna",
    age: 35,
  };

  const handler = {
    get(target, prop) {
      if (prop === "name") {
        return "Mr. " + target[prop];
      }
      return target[prop];
    },
  };

  const proxy = new Proxy(person, handler);

  //Example2:

  var handler1 = {
    get: function (obj, prop) {
      return prop in obj ? obj[prop] : 100;
    },
  };

  var p = new Proxy({}, handler1);
  p.a = 10;
  p.b = null;

  console.log(p.a, p.b); // 10, null
  console.log("c" in p, p.c); // false, 100
  ```

  In the above code, it uses `get` handler which define the behavior of the proxy when an operation is performed on it. These proxies are mainly used for some of the below cross-cutting concerns.

  1. Logging
  2. Authentication or Authorization
  3. Data binding and observables
  4. Function parameter validation

  **Note:** This feature was introduced with ES6.

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.seal()` method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But **values of present properties can still be changed as long as they are writable**. The next level of immutability would be the [`Object.freeze()`](#what-is-a-freeze-method) method. Let's see the below example to understand more about `seal()` method

  ```javascript
  const object = {
    property: "Welcome JS world",
  };
  Object.seal(object);
  object.property = "Welcome to object world";
  console.log(Object.isSealed(object)); // true
  delete object.property; // You cannot delete when sealed
  console.log(object.property); //Welcome to object world
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are the main applications of `Object.seal()` method,

  1. It is used for sealing objects and arrays.
  2. It is used to make properties of an object non-configurable.

  **[⬆ Back to Top](#table-of-contents)**

  If an object is frozen using the `Object.freeze()` method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the `Object.seal()` method then the changes can be made in the existing properties of the object.

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.isSealed()` method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true

  1. If it is not extensible.
  2. If all of its properties are non-configurable.
  3. If it is not removable (but not necessarily non-writable).
     Let's see it in the action

  ```javascript
  const object = {
    property: "Hello, Good morning",
  };

  Object.seal(object); // Using seal() method to seal the object

  console.log(Object.isSealed(object)); // checking whether the object is sealed or not
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.entries()` method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a `for...in` loop. Let's see the functionality of `object.entries()` method in an example,

  ```javascript
  const object = {
    a: "Good morning",
    b: 100,
  };

  for (let [key, value] of Object.entries(object)) {
    console.log(`${key}: ${value}`); // a: 'Good morning'
    // b: 100
  }
  ```

  **Note:** The order is not guaranteed as object defined.

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.values()` method's behavior is similar to `Object.entries()` method but it returns an array of values instead [key,value] pairs.

  ```javascript
  const object = {
    a: "Good morning",
    b: 100,
  };

  for (let value of Object.values(object)) {
    console.log(`${value}`); // 'Good morning \n100'
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Object.keys()` method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,

  ```javascript
  const user = {
    name: "John",
    gender: "male",
    age: 40,
  };

  console.log(Object.keys(user)); //['name', 'gender', 'age']
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.create()` method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.

  ```javascript
  const user = {
    name: "John",
    printInfo: function () {
      console.log(`My name is ${this.name}.`);
    },
  };

  const admin = Object.create(user);

  admin.name = "Nick"; // Remember that "name" is a property set on "admin" but not on "user" object

  admin.printInfo(); // My name is Nick
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A `WeakSet` is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,

  ```javascript
  new WeakSet([iterable]);
  ```

  Let's see the below example to explain it's behavior,

  ```javascript
  var ws = new WeakSet();
  var user = {};
  ws.add(user);
  ws.has(user); // true
  ws.delete(user); // removes user from the set
  ws.has(user); // false, user has been removed
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The main difference is that references to objects in `Set` are strong while references to objects in `WeakSet` are weak. i.e, An object in `WeakSet` can be garbage collected if there is no other reference to it.
  Other differences are:

  1. `Set` can store any value whereas `WeakSet` can store only collections of objects
  2. `WeakSet` does not have size property unlike `Set`
  3. `WeakSet` does not have methods such as clear, keys, values, entries, forEach.
  4. `WeakSet` is not iterable.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of methods available on `WeakSet`,

  1. `add(value)`: A new object is appended with the given value
  2. `delete(value)`: Deletes the value from the collection.
  3. `has(value)`: It returns true if the value is present in the collection, otherwise it returns false.

  Let's see the functionality of all the above methods in an example,

  ```javascript
  var weakSetObject = new WeakSet();
  var firstObject = {};
  var secondObject = {};
  // add(value)
  weakSetObject.add(firstObject);
  weakSetObject.add(secondObject);
  console.log(weakSetObject.has(firstObject)); //true
  weakSetObject.delete(secondObject);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A `WeakMap` object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax looks like the following:

  ```javascript
  new WeakMap([iterable]);
  ```

  Let's see the below example to explain it's behavior,

  ```javascript
  var ws = new WeakMap();
  var user = {};
  ws.set(user);
  ws.has(user); // true
  ws.delete(user); // removes user from the map
  ws.has(user); // false, user has been removed
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The main difference is that references to key objects in `Map` are strong while references to key objects in `WeakMap` are weak. i.e, A key object in `WeakMap` can be garbage collected if there is no other reference to it.
  Other differences are,

  1. `Map` can store any key type whereas `WeakMap` can store only collections of key objects
  2. `WeakMap` does not have size property unlike `Map`
  3. `WeakMap` does not have methods such as clear, keys, values, entries, forEach.
  4. `WeakMap` is not iterable.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of methods available on `WeakMap`,

  1. `set(key, value)`: Sets the value for the key in the `WeakMap` object. Returns the `WeakMap` object.
  2. `delete(key)`: Removes any value associated to the key.
  3. `has(key)`: Returns a Boolean asserting whether a value has been associated to the key in the `WeakMap` object or not.
  4. `get(key)`: Returns the value associated to the key, or undefined if there is none.
     Let's see the functionality of all the above methods in an example,

  ```javascript
  var weakMapObject = new WeakMap();
  var firstObject = {};
  var secondObject = {};
  // set(key, value)
  weakMapObject.set(firstObject, "John");
  weakMapObject.set(secondObject, 100);
  console.log(weakMapObject.has(firstObject)); //true
  console.log(weakMapObject.get(firstObject)); // John
  weakMapObject.delete(secondObject);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `uneval()` is an builtin function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let's see the below example to know more about it's functionality,

  ```javascript
  var a = 1;
  uneval(a); // returns a String containing 1
  uneval(function user() {}); // returns "(function user(){})"
  ```

  The `uneval()` function has been deprecated. It is recommended to use `toString()` for functions and `JSON.stringify()` for other cases.

  ```javascript
  function user() {}
  console.log(user.toString()); // returns "(function user(){})"
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `encodeURI()` function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.

  ```javascript
  var uri = "https://mozilla.org/?x=шеллы";
  var encoded = encodeURI(uri);
  console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `decodeURI()` function is used to decode a Uniform Resource Identifier (URI) previously created by `encodeURI()`.

  ```javascript
  var uri = "https://mozilla.org/?x=шеллы";
  var encoded = encodeURI(uri);
  console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
  try {
    console.log(decodeURI(encoded)); // "https://mozilla.org/?x=шеллы"
  } catch (e) {
    // catches a malformed URI
    console.error(e);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `window` object provides a `print()` method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example,

  ```html
  <input type="button" value="Print" onclick="window.print()" />
  ```

  **Note:** In most browsers, it will block while the print dialog is open.

  **[⬆ Back to Top](#table-of-contents)**

  The `uneval` function returns the source of a given object; whereas the `eval` function does the opposite, by evaluating that source code in a different memory area. Let's see an example to clarify the difference,

  ```javascript
  var msg = uneval(function greeting() {
    return "Hello, Good morning";
  });
  var greeting = eval(msg);
  greeting(); // returns "Hello, Good morning"
  ```

  **[⬆ Back to Top](#table-of-contents)**

  An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,

  ```javascript
  function (optionalParameters) {
    //do something
  }

  const myFunction = function(){ //Anonymous function assigned to a variable
    //do something
  };

  [1, 2, 3].map(function(element){ //Anonymous function used as a callback function
    //do something
  });
  ```

  Let's see the above anonymous function in an example,

  ```javascript
  var x = function (a, b) {
    return a * b;
  };
  var z = x(5, 10);
  console.log(z); // 50
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A local variable takes precedence over a global variable with the same name. Let's see this behavior in an example.

  ```javascript
  var msg = "Good morning";
  function greeting() {
    msg = "Good Evening";
    console.log(msg); // Good Evening
  }
  greeting();
  ```

  **[⬆ Back to Top](#table-of-contents)**

  ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the `get` keyword whereas Setters uses the `set` keyword.

  ```javascript
  var user = {
    firstName: "John",
    lastName: "Abraham",
    language: "en",
    get lang() {
      return this.language;
    },
    set lang(lang) {
      this.language = lang;
    },
  };
  console.log(user.lang); // getter access lang as en
  user.lang = "fr";
  console.log(user.lang); // setter used to set lang as fr
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.defineProperty()` static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let's see an example to know how to define property,

  ```javascript
  const newObject = {};

  Object.defineProperty(newObject, "newProperty", {
    value: 100,
    writable: false,
  });

  console.log(newObject.newProperty); // 100

  newObject.newProperty = 200; // It throws an error in strict mode due to writable setting
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Both have similar results unless you use classes. If you use `get` the property will be defined on the prototype of the object whereas using `Object.defineProperty()` the property will be defined on the instance it is applied to.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of benefits of Getters and Setters,

  1. They provide simpler syntax
  2. They are used for defining computed properties, or accessors in JS.
  3. Useful to provide equivalence relation between properties and methods
  4. They can provide better data quality
  5. Useful for doing things behind the scenes with the encapsulated logic.

  **[⬆ Back to Top](#table-of-contents)**

  Yes, You can use the `Object.defineProperty()` method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,

  ```javascript
  var obj = { counter: 0 };

  // Define getters
  Object.defineProperty(obj, "increment", {
    get: function () {
      this.counter++;
      return this.counter;
    },
  });
  Object.defineProperty(obj, "decrement", {
    get: function () {
      this.counter--;
      return this.counter;
    },
  });

  // Define setters
  Object.defineProperty(obj, "add", {
    set: function (value) {
      this.counter += value;
    },
  });
  Object.defineProperty(obj, "subtract", {
    set: function (value) {
      this.counter -= value;
    },
  });

  obj.add = 10;
  obj.subtract = 5;
  console.log(obj.increment); //6
  console.log(obj.decrement); //5
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `switch case` statement in JavaScript is used for decision making purposes. In a few cases, using the `switch case` statement is going to be more convenient than `if-else` statements. The syntax would be as below,

  ```javascript
  switch (expression)
  {
      case value1:
          statement1;
          break;
      case value2:
          statement2;
          break;
      .
      .
      case valueN:
          statementN;
          break;
      default:
          statementDefault;
  }
  ```

  The above multi-way branch statement provides an easy way to dispatch execution to different parts of code based on the value of the expression.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of conventions should be taken care,

  1. The expression can be of type either number or string.
  2. Duplicate values are not allowed for the expression.
  3. The default statement is optional. If the expression passed to switch does not match with any case value then the statement within default case will be executed.
  4. The break statement is used inside the switch to terminate a statement sequence.
  5. The break statement is optional. But if it is omitted, the execution will continue on into the next case.

  **[⬆ Back to Top](#table-of-contents)**

  A primitive data type is data that has a primitive value (which has no properties or methods). There are 7 types of primitive data types.

  1. string
  2. number
  3. boolean
  4. null
  5. undefined
  6. bigint
  7. symbol

  **[⬆ Back to Top](#table-of-contents)**

  There are 3 possible ways for accessing the property of an object.

  1. **Dot notation:** It uses dot for accessing the properties

  ```javascript
  objectName.property;
  ```

  2. **Square brackets notation:** It uses square brackets for property access

  ```javascript
  objectName["property"];
  ```

  3. **Expression notation:** It uses expression in the square brackets

  ```javascript
  objectName[expression];
  ```

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript functions follow below rules for parameters,

  1. The function definitions do not specify data types for parameters.
  2. Do not perform type checking on the passed arguments.
  3. Do not check the number of arguments received.
     i.e, The below function follows the above rules,

  ```javascript
  function functionName(parameter1, parameter2, parameter3) {
    console.log(parameter1); // 1
  }
  functionName(1);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,

  ```javascript
  try {
    greeting("Welcome");
  } catch (err) {
    console.log(err.name + "<br>" + err.message);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error

  ```javascript
  try {
    eval("greeting('welcome)"); // Missing ' will produce an error
  } catch (err) {
    console.log(err.name);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  There are 7 different types of error names returned from error object,
  | Error Name | Description |
  |---- | ---------
  | `AggregateError` | An error indicating that multiple errors occurred |
  | `EvalError` | An error has occurred in the eval() function |
  | `RangeError` | An error has occurred with a number "out of range" |
  | `ReferenceError` | An error due to an illegal reference|
  | `SyntaxError` | An error due to a syntax error|
  | `TypeError` | An error due to a type error |
  | `URIError` | An error due to encodeURI() |

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of statements used in an error handling,

  1. **try:** This statement is used to test a block of code for errors
  2. **catch:** This statement is used to handle the error
  3. **throw:** This statement is used to create custom errors.
  4. **finally:** This statement is used to execute code after try and catch regardless of the result.

  **[⬆ Back to Top](#table-of-contents)**

  1. **Entry Controlled loops:** In this kind of loop type, the test condition is tested before entering the loop body. For example, For Loop and While Loop comes under this category.
  2. **Exit Controlled Loops:** In this kind of loop type, the test condition is tested or evaluated at the end of the loop body. i.e, the loop body will execute at least once irrespective of test condition true or false. For example, do-while loop comes under this category.

  **[⬆ Back to Top](#table-of-contents)**

  Node.js is a server-side platform built on Chrome's JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google's V8 JavaScript engine and libuv library.

  **[⬆ Back to Top](#table-of-contents)**

  The `Intl` object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Intl.DateTimeFormat` object which is a constructor for objects that enable language-sensitive date and time formatting. Let's see this behavior with an example,

  ```javascript
  var date = new Date(Date.UTC(2019, 07, 07, 3, 0, 0));
  console.log(new Intl.DateTimeFormat("en-GB").format(date)); // 07/08/2019
  console.log(new Intl.DateTimeFormat("en-AU").format(date)); // 07/08/2019
  ```

  **[⬆ Back to Top](#table-of-contents)**

  An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a `next()` method which returns an object with two properties: `value` (the next value in the sequence) and `done` (which is true if the last value in the sequence has been consumed).

  **[⬆ Back to Top](#table-of-contents)**

  Synchronous iteration was introduced in ES6 and it works with below set of components,

  **Iterable:** It is an object which can be iterated over via a method whose key is Symbol.iterator.

  **Iterator:** It is an object returned by invoking `[Symbol.iterator]()` on an iterable. This iterator object wraps each iterated element in an object and returns it via `next()` method one by one.

  **IteratorResult:** It is an object returned by `next()` method. The object contains two properties; the `value` property contains an iterated element and the `done` property determines whether the element is the last element or not.

  Let's demonstrate synchronous iteration with an array as below

  ```javascript
  const iterable = ["one", "two", "three"];
  const iterator = iterable[Symbol.iterator]();
  console.log(iterator.next()); // { value: 'one', done: false }
  console.log(iterator.next()); // { value: 'two', done: false }
  console.log(iterator.next()); // { value: 'three', done: false }
  console.log(iterator.next()); // { value: 'undefined, done: true }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The event loop is a process that continuously monitors both the call stack and the event queue and checks whether or not the call stack is empty. If the call stack is empty and there are pending events in the event queue, the event loop dequeues the event from the event queue and pushes it to the call stack. The call stack executes the event, and any additional events generated during the execution are added to the end of the event queue.

  **Note:** The event loop allows Node.js to perform non-blocking I/O operations, even though JavaScript is single-threaded, by offloading operations to the system kernel whenever possible. Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background.

  **[⬆ Back to Top](#table-of-contents)**

  Call Stack is a data structure for javascript interpreters to keep track of function calls(creates execution context) in the program. It has two major actions,

  1. Whenever you call a function for its execution, you are pushing it to the stack.
  2. Whenever the execution is completed, the function is popped out of the stack.

  Let's take an example and it's state representation in a diagram format

  ```javascript
  function hungry() {
    eatFruits();
  }
  function eatFruits() {
    return "I'm eating fruits";
  }

  // Invoke the `hungry` function
  hungry();
  ```

  The above code processed in a call stack as below,

  3. Add the `hungry()` function to the call stack list and execute the code.
  4. Add the `eatFruits()` function to the call stack list and execute the code.
  5. Delete the `eatFruits()` function from our call stack list.
  6. Delete the `hungry()` function from the call stack list since there are no items anymore.

  ![Screenshot](images/call-stack.png)

  **[⬆ Back to Top](#table-of-contents)**

  The event queue follows the queue data structure. It stores async callbacks to be added to the call stack. It is also known as the Callback Queue or Macrotask Queue.

  Whenever the call stack receives an async function, it is moved into the Web API. Based on the function, Web API executes it and awaits the result. Once it is finished, it moves the callback into the event queue (the callback of a promise is moved into the microtask queue).

  The event loop constantly checks whether or not the call stack is empty. Once the call stack is empty and there is a callback in the event queue, the event loop moves the callback into the call stack. But if there is a callback in the microtask queue as well, it is moved first. The microtask queue has a higher priority than the event queue.

  **[⬆ Back to Top](#table-of-contents)**

  A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let's define admin decorator for user class at design time,

  ```javascript
  function admin(isAdmin) {
     return function(target) {
         target.isAdmin = isAdmin;
     }
  }

  @admin(true)
  class User() {
  }
  console.log(User.isAdmin); //true

   @admin(false)
   class User() {
   }
   console.log(User.isAdmin); //false
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of properties available on the `Intl` object,

  1. **Collator:** These are the objects that enable language-sensitive string comparison.
  2. **DateTimeFormat:** These are the objects that enable language-sensitive date and time formatting.
  3. **ListFormat:** These are the objects that enable language-sensitive list formatting.
  4. **NumberFormat:** Objects that enable language-sensitive number formatting.
  5. **PluralRules:** Objects that enable plural-sensitive formatting and language-specific rules for plurals.
  6. **RelativeTimeFormat:** Objects that enable language-sensitive relative time formatting.

  **[⬆ Back to Top](#table-of-contents)**

  The unary(+) operator is used to convert a variable to a number.If the variable cannot be converted, it will still become a number but with the value NaN. Let's see this behavior in an action.

  ```javascript
  var x = "100";
  var y = +x;
  console.log(typeof x, typeof y); // string, number

  var a = "Hello";
  var b = +a;
  console.log(typeof a, typeof b, b); // string, number, NaN
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `sort()` method is used to sort the elements of an array in place and returns the sorted array. The default sort order is ascending, based on the string Unicode order. The example usage would be as below,

  ```javascript
  var months = ["Aug", "Sep", "Jan", "June"];
  months.sort();
  console.log(months); //  ["Aug", "Jan", "June", "Sep"]
  ```

  **Beware:** `sort()` is changing the original array.

  **[⬆ Back to Top](#table-of-contents)**

  The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value.

  Let's take an example to see the usage of compareFunction,

  ```javascript
  let numbers = [1, 2, 5, 3, 4];
  numbers.sort((a, b) => b - a);
  console.log(numbers); // [5, 4, 3, 2, 1]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `reverse()` method to reverse the elements in an array. This method is useful to sort an array in descending order. Let's see the usage of `reverse()` method in an example,

  ```javascript
  let numbers = [1, 2, 5, 3, 4];
  numbers.sort((a, b) => b - a);
  numbers.reverse();
  console.log(numbers); // [1, 2, 3, 4 ,5]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use `Math.min` and `Math.max` methods on array variables to find the minimum and maximum elements within an array. Let's create two functions to find the min and max value with in an array,

  ```javascript
  var marks = [50, 20, 70, 60, 45, 30];
  function findMin(arr) {
    return Math.min.apply(null, arr);
  }
  function findMax(arr) {
    return Math.max.apply(null, arr);
  }

  console.log(findMin(marks));
  console.log(findMax(marks));
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let's create those functions to find min and max values,

  ```javascript
  var marks = [50, 20, 70, 60, 45, 30];
  function findMin(arr) {
    var length = arr.length;
    var min = Infinity;
    while (length--) {
      if (arr[length] < min) {
        min = arr[length];
      }
    }
    return min;
  }

  function findMax(arr) {
    var length = arr.length;
    var max = -Infinity;
    while (length--) {
      if (arr[length] > max) {
        max = arr[length];
      }
    }
    return max;
  }

  console.log(findMin(marks));
  console.log(findMax(marks));
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one. Since there is no action with an empty statement you might think that it's usage is quite less, but the empty statement is occasionally useful when you want to create a loop that has an empty body. For example, you can initialize an array with zero values as below,

  ```javascript
  // Initialize an array a
  for (let i = 0; i < a.length; a[i++] = 0);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `import.meta` object which is a meta-property exposing context-specific meta data to a JavaScript module. It contains information about the current module, such as the module's URL. In browsers, you might get different meta data than NodeJS.

  ```javascript
  <script type="module" src="welcome-module.js"></script>;
  console.log(import.meta); // { url: "file:///home/user/welcome-module.js" }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below,

  ```javascript
  var x = 1;
  x = (x++, x);

  console.log(x); // 2
  ```

  **[⬆ Back to Top](#table-of-contents)**

  It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a `for` loop. For example, the below for loop uses multiple expressions in a single location using comma operator,

  ```javascript
  for (var a = 0, b =10; a <= 10; a++, b--)
  ```

  You can also use the comma operator in a return statement where it processes before returning.

  ```javascript
  function myFunction() {
    var a = 1;
    return (a += 10), a; // 11
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes
  and many other features, and compiles to plain JavaScript. Angular is built entirely in TypeScript and it is used as the primary language there. You can install it globally as

  ```bash
  npm install -g typescript
  ```

  Let's see a simple example of TypeScript usage,

  ```typescript
  function greeting(name: string): string {
    return "Hello, " + name;
  }

  let user = "Sudheer";

  console.log(greeting(user));
  ```

  The greeting method allows only string type as argument.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of differences between javascript and typescript,

  | feature             | typescript                            | javascript                                      |
  | ------------------- | ------------------------------------- | ----------------------------------------------- |
  | Language paradigm   | Object oriented programming language  | Multi-paradigm language                         |
  | Typing support      | Supports static typing                | Dynamic typing                                  |
  | Modules             | Supported                             | Not supported                                   |
  | Interface           | It has interfaces concept             | Doesn't support interfaces                      |
  | Optional parameters | Functions support optional parameters | No support of optional parameters for functions |

  **[⬆ Back to Top](#table-of-contents)**

  Below are some of the advantages of typescript over javascript,

  1. TypeScript is able to find compile time errors at the development time only and it makes sures less runtime errors. Whereas javascript is an interpreted language.
  2. TypeScript is strongly-typed or supports static typing which allows for checking type correctness at compile time. This is not available in javascript.
  3. TypeScript compiler can compile the .ts files into ES3,ES4 and ES5 unlike ES6 features of javascript which may not be supported in some browsers.

  **[⬆ Back to Top](#table-of-contents)**

  An object initializer is an expression that describes the initialization of an Object. The syntax for this expression is represented as a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). This is also known as literal notation. It is one of the ways to create an object.

  ```javascript
  var initObject = { a: "John", b: 50, c: {} };

  console.log(initObject.a); // John
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below,

  ```javascript
  class Employee {
    constructor() {
      this.name = "John";
    }
  }

  var employeeObject = new Employee();

  console.log(employeeObject.name); // John
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The "constructor" in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a `SyntaxError` error.

  ```javascript
   class Employee {
     constructor() {
       this.name = "John";
     }
     constructor() {   //  Uncaught SyntaxError: A class may only have one constructor
       this.age = 30;
     }
   }

   var employeeObject = new Employee();

   console.log(employeeObject.name);
  ```

  This constructor is called by using the special function call `new` (see example above).

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `super` keyword to call the constructor of a parent class. Remember that `super()` must be called before using `this` reference. Otherwise it will cause a reference error. Let's the usage of it,

  ```javascript
  class Square extends Rectangle {
    constructor(length) {
      super(length, length);
      this.name = "Square";
    }

    get area() {
      return this.width * this.height;
    }

    set area(value) {
      this.area = value;
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Object.getPrototypeOf(obj)` method to return the prototype of the specified object. i.e. The value of the internal `prototype` property. If there are no inherited properties then `null` value is returned.

  ```javascript
  const newPrototype = {};
  const newObject = Object.create(newPrototype);

  console.log(Object.getPrototypeOf(newObject) === newPrototype); // true
  ```

  **[⬆ Back to Top](#table-of-contents)**

  In ES5, it will throw a TypeError exception if the obj parameter isn't an object. Whereas in ES2015, the parameter will be coerced to an `Object`.

  ```javascript
  // ES5
  Object.getPrototypeOf("James"); // TypeError: "James" is not an object
  // ES2015
  Object.getPrototypeOf("James"); // String.prototype
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Object.setPrototypeOf()` method that sets the prototype (i.e., the internal `Prototype` property) of a specified object to another object or null. For example, if you want to set prototype of a square object to rectangle object would be as follows,

  ```javascript
  Object.setPrototypeOf(Square.prototype, Rectangle.prototype);
  Object.setPrototypeOf({}, null);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.isExtensible()` method is used to determine if an object is extendable or not. i.e, Whether it can have new properties added to it or not.

  ```javascript
  const newObject = {};
  console.log(Object.isExtensible(newObject)); //true
  ```

  **Note:** By default, all the objects are extendable. i.e, The new properties can be added or modified.

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.preventExtensions()` method is used to prevent new properties from ever being added to an object. In other words, it prevents future extensions to the object. Let's see the usage of this property,

  ```javascript
  const newObject = {};
  Object.preventExtensions(newObject); // NOT extendable

  try {
    Object.defineProperty(newObject, "newProperty", {
      // Adding new property
      value: 100,
    });
  } catch (e) {
    console.log(e); // TypeError: Cannot define property newProperty, object is not extensible
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can mark an object non-extensible in 3 ways,

  1. `Object.preventExtensions`
  2. `Object.seal`
  3. `Object.freeze`

  ```javascript
  var newObject = {};

  Object.preventExtensions(newObject); // Prevent objects are non-extensible
  Object.isExtensible(newObject); // false

  var sealedObject = Object.seal({}); // Sealed objects are non-extensible
  Object.isExtensible(sealedObject); // false

  var frozenObject = Object.freeze({}); // Frozen objects are non-extensible
  Object.isExtensible(frozenObject); // false
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `Object.defineProperties()` method is used to define new or modify existing properties directly on an object and returning the object. Let's define multiple properties on an empty object,

  ```javascript
  const newObject = {};

  Object.defineProperties(newObject, {
    newProperty1: {
      value: "John",
      writable: true,
    },
    newProperty2: {},
  });
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open-source JavaScript software tech stack available for building dynamic web apps where you can write both the server-side and client-side halves of the web project entirely in JavaScript.

  **[⬆ Back to Top](#table-of-contents)**

  Obfuscation is the deliberate act of creating obfuscated javascript code(i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it.
  Let's see the below function before Obfuscation,

  ```javascript
  function greeting() {
    console.log("Hello, welcome to JS world");
  }
  ```

  And after the code Obfuscation, it would be appeared as below,

  ```javascript
  eval(
    (function (p, a, c, k, e, d) {
      e = function (c) {
        return c;
      };
      if (!"".replace(/^/, String)) {
        while (c--) {
          d[c] = k[c] || c;
        }
        k = [
          function (e) {
            return d[e];
          },
        ];
        e = function () {
          return "\\w+";
        };
        c = 1;
      }
      while (c--) {
        if (k[c]) {
          p = p.replace(new RegExp("\\b" + e(c) + "\\b", "g"), k[c]);
        }
      }
      return p;
    })(
      "2 1(){0.3('4, 7 6 5 8')}",
      9,
      9,
      "console|greeting|function|log|Hello|JS|to|welcome|world".split("|"),
      0,
      {}
    )
  );
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are the few reasons for Obfuscation,

  1. The Code size will be reduced. So data transfers between server and client will be fast.
  2. It hides the business logic from outside world and protects the code from others
  3. Reverse engineering is highly difficult
  4. The download time will be reduced

  **[⬆ Back to Top](#table-of-contents)**

  Minification is the process of removing all unnecessary characters(empty spaces are removed) and variables will be renamed without changing it's functionality. It is also a type of obfuscation .

  **[⬆ Back to Top](#table-of-contents)**

  Normally it is recommended to use minification for heavy traffic and intensive requirements of resources. It reduces file sizes with below benefits,

  1. Decreases loading times of a web page
  2. Saves bandwidth usages

  **[⬆ Back to Top](#table-of-contents)**

  Below are the main differences between obfuscation and encryption,

  | Feature            | Obfuscation                                     | Encryption                                                              |
  | ------------------ | ----------------------------------------------- | ----------------------------------------------------------------------- |
  | Definition         | Changing the form of any data in any other form | Changing the form of information to an unreadable format by using a key |
  | A key to decode    | It can be decoded without any key               | It is required                                                          |
  | Target data format | It will be converted to a complex form          | Converted into an unreadable format                                     |

  **[⬆ Back to Top](#table-of-contents)**

  There are many online/offline tools to minify the javascript files,

  1. Google's Closure Compiler
  2. UglifyJS2
  3. jsmin
  4. javascript-minifier.com/
  5. prettydiff.com

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted.
  Let's perform user login in an html form,

  ```html
  <form name="myForm" onsubmit="return validateForm()" method="post">
    User name:
    <input type="text" name="uname" />
    <input type="submit" value="Submit" />
  </form>
  ```

  And the validation on user login is below,

  ```javascript
  function validateForm() {
    var x = document.forms["myForm"]["uname"].value;
    if (x == "") {
      alert("The username shouldn't be empty");
      return false;
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can perform HTML form validation automatically without using javascript. The validation enabled by applying the `required` attribute to prevent form submission when the input is empty.

  ```html
  <form method="post">
    <input type="text" name="uname" required />
    <input type="submit" value="Submit" />
  </form>
  ```

  **Note:** Automatic form validation does not work in Internet Explorer 9 or earlier.

  **[⬆ Back to Top](#table-of-contents)**

  The below DOM methods are available for constraint validation on an invalid input,

  1. `checkValidity()`: It returns true if an input element contains valid data.
  2. `setCustomValidity()`: It is used to set the `validationMessage` property of an input element.
     Let's take an user login form with DOM validations

  ```javascript
  function myFunction() {
    var userName = document.getElementById("uname");
    if (!userName.checkValidity()) {
      document.getElementById("message").innerHTML =
        userName.validationMessage;
    } else {
      document.getElementById("message").innerHTML =
        "Entered a valid username";
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of some of the constraint validation DOM properties available,

  1. `validity`: It provides a list of boolean properties related to the validity of an input element.
  2. `validationMessage`: It displays the message when the validity is false.
  3. `willValidate`: It indicates if an input element will be validated or not.

  **[⬆ Back to Top](#table-of-contents)**

  The validity property of an input element provides a set of properties related to the validity of data.

  1. `customError`: It returns true, if a custom validity message is set.
  2. `patternMismatch`: It returns true, if an element's value does not match its pattern attribute.
  3. `rangeOverflow`: It returns true, if an element's value is greater than its max attribute.
  4. `rangeUnderflow`: It returns true, if an element's value is less than its min attribute.
  5. `stepMismatch`: It returns true, if an element's value is invalid according to step attribute.
  6. `tooLong`: It returns true, if an element's value exceeds its maxLength attribute.
  7. `typeMismatch`: It returns true, if an element's value is invalid according to type attribute.
  8. `valueMissing`: It returns true, if an element with a required attribute has no value.
  9. `valid`: It returns true, if an element's value is valid.

  **[⬆ Back to Top](#table-of-contents)**

  If an element's value is greater than its max attribute then the `rangeOverflow` property is true. For example, the below form submission throws an error if the value is more than 100,

  ```html
  <input id="age" type="number" max="100" />
  <button onclick="myOverflowFunction()">OK</button>
  ```

  ```javascript
  function myOverflowFunction() {
    if (document.getElementById("age").validity.rangeOverflow) {
      alert("The mentioned age is not allowed");
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  No, javascript does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object,

  ```javascript
  var DaysEnum = Object.freeze({"monday":1, "tuesday":2, "wednesday":3, ...})
  ```

  **[⬆ Back to Top](#table-of-contents)**

  An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but typescript provides built-in enum support.

  ```javascript
  enum Color {
   RED, GREEN, BLUE
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Object.getOwnPropertyNames()` method which returns an array of all properties found directly in a given object. Let's see the usage of this in an example below:

  ```javascript
  const newObject = {
    a: 1,
    b: 2,
    c: 3,
  };

  console.log(Object.getOwnPropertyNames(newObject));
  ["a", "b", "c"];
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Object.getOwnPropertyDescriptors()` method which returns all own property descriptors of a given object. The example usage of this method is below,

  ```javascript
  const newObject = {
    a: 1,
    b: 2,
    c: 3,
  };
  const descriptorsObject = Object.getOwnPropertyDescriptors(newObject);
  console.log(descriptorsObject.a.writable); //true
  console.log(descriptorsObject.a.configurable); //true
  console.log(descriptorsObject.a.enumerable); //true
  console.log(descriptorsObject.a.value); // 1
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A property descriptor is a record which has the following attributes

  1. `value`: The value associated with the property
  2. `writable`: Determines whether the value associated with the property can be changed or not
  3. `configurable`: Returns true if the type of this property descriptor can be changed and if the property can be deleted from the corresponding object.
  4. `enumerable`: Determines whether the property appears during enumeration of the properties on the corresponding object or not.
  5. `set`: A function which serves as a setter for the property
  6. `get`: A function which serves as a getter for the property

  **[⬆ Back to Top](#table-of-contents)**

  The `extends` keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,

  ```javascript
  class ChildClass extends ParentClass { ... }
  ```

  Let's take an example of Square subclass from Polygon parent class,

  ```javascript
  class Square extends Rectangle {
    constructor(length) {
      super(length, length);
      this.name = "Square";
    }

    get area() {
      return this.width * this.height;
    }

    set area(value) {
      this.area = value;
    }
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `window.location.href` property will be helpful to modify the url but it reloads the page. HTML5 introduced the `history.pushState()` and `history.replaceState()` methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below,

  ```javascript
  window.history.pushState("page2", "Title", "/page2.html");
  ```

  This mechanism is used by routing libraries of frameworks like React and Angular in order to simulate the behaviour of a multi-page-website, even though they are only SPA (Single Page Applications).

  **[⬆ Back to Top](#table-of-contents)**

  The `Array#includes()` method is used to determine whether an array includes a particular value among its entries by returning either true or false. Let's see an example to find an element(numeric and string) within an array.

  ```javascript
  var numericArray = [1, 2, 3, 4];
  console.log(numericArray.includes(3)); // true

  var stringArray = ["green", "yellow", "blue"];
  console.log(stringArray.includes("blue")); //true
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use length and every method of arrays to compare two scalars (compared directly using `===`) arrays. The combination of these expressions can give the expected result,

  ```javascript
  const arrayFirst = [1, 2, 3, 4, 5];
  const arraySecond = [1, 2, 3, 4, 5];
  console.log(
    arrayFirst.length === arraySecond.length &&
      arrayFirst.every((value, index) => value === arraySecond[index])
  ); // true
  ```

  If you would like to compare arrays irrespective of order then you should sort them before,

  ```javascript
  const arrayFirst = [2, 3, 1, 4, 5];
  const arraySecond = [1, 2, 3, 4, 5];
  console.log(
    arrayFirst.length === arraySecond.length &&
      arrayFirst
        .sort()
        .every((value, index) => value === arraySecond[index])
  ); //true
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `new URL()` object accepts the url string and `searchParams` property of this object can be used to access the get parameters.

  ```javascript
  let urlString = "http://www.some-domain.com/about.html?x=1&y=2&z=3"; //window.location.href
  let url = new URL(urlString);
  let parameterZ = url.searchParams.get("z");
  console.log(parameterZ); // 3
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `Number.prototype.toLocaleString()` method which returns a string with a language-sensitive representation such as thousand separator, currency etc. of this number.

  ```javascript
  function convertToThousandFormat(x) {
    return x.toLocaleString(); // 12,345.679
  }

  console.log(convertToThousandFormat(12345.6789));
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Both are totally unrelated programming languages and no relation between them. Java is statically typed, compiled, runs on its own VM. Whereas JavaScript is dynamically typed, interpreted, and runs in a browser and nodejs environments. Let's see the major differences in a tabular format,
  | Feature | Java | JavaScript |
  |---- | ---- | -----
  | Typed | It's a strongly typed language | It's a dynamic typed language |
  | Paradigm | Object oriented programming | Prototype based programming |
  | Scoping | Block scoped | Function-scoped, block scoped since ES6 |
  | Concurrency | Thread based | event based |

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript doesn’t support namespaces by default. So if you create any element (function, method, object, variable) then it becomes global and pollutes the global namespace. Let's take an example of defining two functions without any namespace,

  ```javascript
  function func1() {
    console.log("This is a first definition");
  }
  function func1() {
    console.log("This is a second definition");
  }
  func1(); // This is a second definition
  ```

  It always calls the second function definition. In this case, namespaces will solve the name collision problem.

  **[⬆ Back to Top](#table-of-contents)**

  Even though JavaScript lacks namespaces, we can use Objects, an IIFE (Immediately Invoked Function Expression) or `let`/`const` to create namespaces.

  1. **Using Object Literal Notation:** Let's wrap variables and functions inside an Object literal which acts as a namespace. After that you can access them using object notation

     ```javascript
     var namespaceOne = {
         function func1() {
             console.log("This is a first definition");
         }
     }
     var namespaceTwo = {
           function func1() {
               console.log("This is a second definition");
           }
       }
     namespaceOne.func1(); // This is a first definition
     namespaceTwo.func1(); // This is a second definition
     ```

  2. **Using IIFE (Immediately invoked function expression):** The outer pair of parentheses of IIFE creates a local scope for all the code inside of it and makes the anonymous function a function expression. Due to that, you can create the same function in two different function expressions to act as a namespace.

     ```javascript
     (function () {
       function fun1() {
         console.log("This is a first definition");
       }
       fun1();
     })();

     (function () {
       function fun1() {
         console.log("This is a second definition");
       }
       fun1();
     })();
     ```

  3. **Using a block and a let/const declaration:** In ECMAScript 6, you can simply use a block and a let declaration to restrict the scope of a variable to a block.

     ```javascript
     {
       let myFunction = function fun1() {
         console.log("This is a first definition");
       };
       myFunction();
     }
     //myFunction(): ReferenceError: myFunction is not defined.

     {
       let myFunction = function fun1() {
         console.log("This is a second definition");
       };
       myFunction();
     }
     //myFunction(): ReferenceError: myFunction is not defined.
     ```

  **[⬆ Back to Top](#table-of-contents)**

  Initially iFrame needs to be accessed using either `document.getElementBy` or `window.frames`. After that `contentWindow` property of iFrame gives the access for targetFunction

  ```javascript
  document.getElementById("targetFrame").contentWindow.targetFunction();
  window.frames[0].frameElement.contentWindow.targetFunction(); // Accessing iframe this way may not work in latest versions chrome and firefox
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `getTimezoneOffset` method of the date object. This method returns the time zone difference, in minutes, from current locale (host system settings) to UTC

  ```javascript
  var offset = new Date().getTimezoneOffset();
  console.log(offset); // -480
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can create both link and script elements in the DOM and append them as child to head tag. Let's create a function to add script and style resources as below,

  ```javascript
  function loadAssets(filename, filetype) {
    if (filetype == "css") {
      // External CSS file
      var fileReference = document.createElement("link");
      fileReference.setAttribute("rel", "stylesheet");
      fileReference.setAttribute("type", "text/css");
      fileReference.setAttribute("href", filename);
    } else if (filetype == "js") {
      // External JavaScript file
      var fileReference = document.createElement("script");
      fileReference.setAttribute("type", "text/javascript");
      fileReference.setAttribute("src", filename);
    }
    if (typeof fileReference != "undefined")
      document.getElementsByTagName("head")[0].appendChild(fileReference);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  If you want to access any element in an HTML page, you need to start with accessing the document object. Later you can use any of the below methods to find the HTML element,

  1. `document.getElementById(id)`: It finds an element by Id
  2. `document.getElementsByTagName(name)`: It finds an element by tag name (returns an node list)
  3. `document.getElementsByClassName(name)`: It finds an element by class name (returns an node list)
  4. `document.querySelector(cssSelector)`: It finds an element by css selector
  5. `document.querySelectorAll(cssSelector)`: It finds all elements by css selector (returns a node list)

  **[⬆ Back to Top](#table-of-contents)**

  jQuery is a popular cross-browser JavaScript library that provides Document Object Model (DOM) traversal, event handling, animations and AJAX interactions by minimizing the discrepancies across browsers. It is widely famous with its philosophy of “Write less, do more”. For example, you can display welcome message on the page load using jQuery as below,

  ```javascript
  $(document).ready(function () {
    // It selects the document and apply the function on page load
    alert("Welcome to jQuery world");
  });
  ```

  **Note:** You can download it from jquery's official site or install it from CDNs, like google.

  **[⬆ Back to Top](#table-of-contents)**

  V8 is an open source high-performance JavaScript engine used by the Google Chrome browser, written in C++. It is also being used in the node.js project. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors.
  **Note:** It can run standalone, or can be embedded into any C++ application.

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.

  ```javascript
  let age = 50; // age is a number now
  age = "old"; // age is a string now
  age = true; // age is a boolean
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `void` operator evaluates the given expression and then returns `undefined` (i.e, without returning value). The syntax would be as below,

  ```javascript
  void expression;
  void expression;
  ```

  Let's display a message without any redirection or reload

  ```javascript
  <a href="javascript:void(alert('Welcome to JS world'))">
    Click here to see a message
  </a>
  ```

  **Note:** This operator is often used to obtain the undefined primitive value, using `void(0)`. Also it can be used to call asynchronous functions without waiting for the result.

  **[⬆ Back to Top](#table-of-contents)**

  The cursor can be set to wait in JavaScript by using the property `cursor`. Let's perform this behavior on page load using the below function.

  ```javascript
  function myFunction() {
    window.document.body.style.cursor = "wait";
  }
  ```

  and this function invoked on page load

  ```html
  <body onload="myFunction()"></body>
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools,

  ```javascript
  for (;;) {}
  while (true) {}
  ```

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript's with statement was intended to provide a shorthand for writing recurring accesses to objects. So it can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. Let's take an example where it is used to avoid redundancy when accessing an object several times.

  ```javascript
  a.b.c.greeting = "welcome";
  a.b.c.age = 32;
  ```

  Using `with` it turns this into:

  ```javascript
  with (a.b.c) {
    greeting = "welcome";
    age = 32;
  }
  ```

  But this `with` statement creates performance problems since one cannot predict whether an argument will refer to a real variable or to a property inside the with argument.

  **[⬆ Back to Top](#table-of-contents)**

  ```javascript
  for (var i = 0; i < 4; i++) {
    // global scope
    setTimeout(() => console.log(i));
  }

  for (let i = 0; i < 4; i++) {
    // block scope
    setTimeout(() => console.log(i));
  }
  ```

  The output of the above for loops is 4 4 4 4 and 0 1 2 3

  **Explanation:** Due to the event queue/loop of javascript, the `setTimeout` callback function is called after the loop has been executed. Since the variable i is declared with the `var` keyword it became a global variable and the value was equal to 4 using iteration when the time `setTimeout` function is invoked. Hence, the output of the second loop is `4 4 4 4`.

  Whereas in the second loop, the variable i is declared as the `let` keyword it becomes a block scoped variable and it holds a new value(0, 1 ,2 3) for each iteration. Hence, the output of the first loop is `0 1 2 3`.

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of some new features of ES6,

  1. Support for constants or immutable variables
  2. Block-scope support for variables, constants and functions
  3. Arrow functions
  4. Default parameters
  5. Rest and Spread Parameters
  6. Template Literals
  7. Multi-line Strings
  8. Destructuring Assignment
  9. Enhanced Object Literals
  10. Promises
  11. Classes
  12. Modules

  **[⬆ Back to Top](#table-of-contents)**

  ES6 is the sixth edition of the javascript language and it was released in June 2015. It was initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015. Almost all the modern browsers support ES6 but for the old browsers there are many transpilers, like Babel.js etc.

  **[⬆ Back to Top](#table-of-contents)**

  No, you cannot redeclare let and const variables. If you do, it throws below error

  ```bash
  Uncaught SyntaxError: Identifier 'someVariable' has already been declared
  ```

  **Explanation:** The variable declaration with `var` keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let's take an example of re-declaring variables in the same scope for both var and let/const variables.

  ```javascript
  var name = "John";
  function myFunc() {
    var name = "Nick";
    var name = "Abraham"; // Re-assigned in the same function block
    alert(name); // Abraham
  }
  myFunc();
  alert(name); // John
  ```

  The block-scoped multi-declaration throws syntax error,

  ```javascript
  let name = "John";
  function myFunc() {
    let name = "Nick";
    let name = "Abraham"; // Uncaught SyntaxError: Identifier 'name' has already been declared
    alert(name);
  }

  myFunc();
  alert(name);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  No, the `const` variable doesn't make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can't assign another value later)

  ```javascript
  const userList = [];
  userList.push("John"); // Can mutate even though it can't re-assign
  console.log(userList); // ['John']
  ```

  **[⬆ Back to Top](#table-of-contents)**

  In ES5, we need to depend on logical OR operators to handle default values of function parameters. Whereas in ES6, Default function parameters feature allows parameters to be initialized with default values if no value or undefined is passed. Let's compare the behavior with an examples,

  ```javascript
  //ES5
  var calculateArea = function (height, width) {
    height = height || 50;
    width = width || 60;

    return width * height;
  };
  console.log(calculateArea()); //300
  ```

  The default parameters makes the initialization more simpler,

  ```javascript
  //ES6
  var calculateArea = function (height = 50, width = 60) {
    return width * height;
  };

  console.log(calculateArea()); //300
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick (`) character instead of double or single quotes.
  In ES6, this feature enables using dynamic expressions as below,

  ```javascript
  var greeting = `Welcome to JS World, Mr. ${firstName} ${lastName}.`;
  ```

  In ES5, you need break string like below,

  ```javascript
  var greeting = 'Welcome to JS World, Mr. ' + firstName + ' ' + lastName.`
  ```

  **Note:** You can use multi-line strings and string interpolation features with template literals.

  **[⬆ Back to Top](#table-of-contents)**

  In ES5, you would have to use newline escape characters('\\n') and concatenation symbols(+) in order to get multi-line strings.

  ```javascript
  console.log("This is string sentence 1\n" + "This is string sentence 2");
  ```

  Whereas in ES6, You don't need to mention any newline sequence character,

  ```javascript
  console.log(`This is string sentence
  'This is string sentence 2`);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The nesting template is a feature supported within template literals syntax to allow inner backticks inside a placeholder ${ } within the template. For example, the below nesting template is used to display the icons based on user permissions whereas outer template checks for platform type,

  ```javascript
  const iconStyles = `icon ${
    isMobilePlatform()
      ? ""
      : `icon-${user.isAuthorized ? "submit" : "disabled"}`
  }`;
  ```

  You can write the above use case without nesting template features as well. However, the nesting template feature is more compact and readable.

  ```javascript
  //Without nesting templates
  const iconStyles = `icon ${
    isMobilePlatform()
      ? ""
      : user.isAuthorized
      ? "icon-submit"
      : "icon-disabled"
  }`;
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Tagged templates are the advanced form of templates in which tags allow you to parse template literals with a function. The tag function accepts the first parameter as an array of strings and remaining parameters as expressions. This function can also return manipulated strings based on parameters. Let's see the usage of this tagged template behavior of an IT professional skill set in an organization,

  ```javascript
  var user1 = "John";
  var skill1 = "JavaScript";
  var experience1 = 15;

  var user2 = "Kane";
  var skill2 = "JavaScript";
  var experience2 = 5;

  function myInfoTag(strings, userExp, experienceExp, skillExp) {
    var str0 = strings[0]; // "Mr/Ms. "
    var str1 = strings[1]; // " is a/an "
    var str2 = strings[2]; // "in"

    var expertiseStr;
    if (experienceExp > 10) {
      expertiseStr = "expert developer";
    } else if (skillExp > 5 && skillExp <= 10) {
      expertiseStr = "senior developer";
    } else {
      expertiseStr = "junior developer";
    }

    return `${str0}${userExp}${str1}${expertiseStr}${str2}${skillExp}`;
  }

  var output1 = myInfoTag`Mr/Ms. ${user1} is a/an ${experience1} in ${skill1}`;
  var output2 = myInfoTag`Mr/Ms. ${user2} is a/an ${experience2} in ${skill2}`;

  console.log(output1); // Mr/Ms. John is a/an expert developer in JavaScript
  console.log(output2); // Mr/Ms. Kane is a/an junior developer in JavaScript
  ```

  **[⬆ Back to Top](#table-of-contents)**

  ES6 provides a raw strings feature using the `String.raw()` method which is used to get the raw string form of template strings. This feature allows you to access the raw strings as they were entered, without processing escape sequences. For example, the usage would be as below,

  ```javascript
  var calculationString = String.raw`The sum of numbers is \n${
    1 + 2 + 3 + 4
  }!`;
  console.log(calculationString); // The sum of numbers is \n10!
  ```

  If you don't use raw strings, the newline character sequence will be processed by displaying the output in multiple lines

  ```javascript
  var calculationString = `The sum of numbers is \n${1 + 2 + 3 + 4}!`;
  console.log(calculationString);
  // The sum of numbers is
  // 10!
  ```

  Also, the raw property is available on the first argument to the tag function

  ```javascript
  function tag(strings) {
    console.log(strings.raw[0]);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables.
  Let's get the month values from an array using destructuring assignment

  ```javascript
  var [one, two, three] = ["JAN", "FEB", "MARCH"];

  console.log(one); // "JAN"
  console.log(two); // "FEB"
  console.log(three); // "MARCH"
  ```

  and you can get user properties of an object using destructuring assignment,

  ```javascript
  var { name, age } = { name: "John", age: 32 };

  console.log(name); // John
  console.log(age); // 32
  ```

  **[⬆ Back to Top](#table-of-contents)**

  A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let's take an example for both arrays and object use cases,

  **Arrays destructuring:**

  ```javascript
  var x, y, z;

  [x = 2, y = 4, z = 6] = [10];
  console.log(x); // 10
  console.log(y); // 4
  console.log(z); // 6
  ```

  **Objects destructuring:**

  ```javascript
  var { x = 2, y = 4, z = 6 } = { x: 10 };

  console.log(x); // 10
  console.log(y); // 4
  console.log(z); // 6
  ```

  **[⬆ Back to Top](#table-of-contents)**

  If you don't use destructuring assignment, swapping two values requires a temporary variable. Whereas using a destructuring feature, two variable values can be swapped in one destructuring expression. Let's swap two number variables in array destructuring assignment,

  ```javascript
  var x = 10,
    y = 20;

  [x, y] = [y, x];
  console.log(x); // 20
  console.log(y); // 10
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Object literals make it easy to quickly create objects with properties inside the curly braces. For example, it provides shorter syntax for common object property definition as below.

  ```javascript
  //ES6
  var x = 10,
    y = 20;
  obj = { x, y };
  console.log(obj); // {x: 10, y:20}
  //ES5
  var x = 10,
    y = 20;
  obj = { x: x, y: y };
  console.log(obj); // {x: 10, y:20}
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The dynamic imports using `import()` function syntax allows us to load modules on demand by using promises or the async/await syntax. Currently this feature is in [stage4 proposal](https://github.com/tc39/proposal-dynamic-import). The main advantage of dynamic imports is reduction of our bundle's sizes, the size/payload response of our requests and overall improvements in the user experience.
  The syntax of dynamic imports would be as below,

  ```javascript
  import("./Module").then((Module) => Module.method());
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are some of the use cases of using dynamic imports over static imports,

  1. Import a module on-demand or conditionally. For example, if you want to load a polyfill on legacy browser

     ```javascript
     if (isLegacyBrowser()) {
         import(···)
         .then(···);
     }
     ```

  2. Compute the module specifier at runtime. For example, you can use it for internationalization.

     ```javascript
     import(`messages_${getLocale()}.js`).then(···);
     ```

  3. Import a module from within a regular script instead a module.

  **[⬆ Back to Top](#table-of-contents)**

  Typed arrays are array-like objects from ECMAScript 6 API for handling binary data. JavaScript provides 12 Typed array types,

  1. Int8Array: An array of 8-bit signed integers
  2. Uint8Array: An array of 8-bit unsigned integers
  3. Uint8ClampedArray: An array of 8-bit unsigned integers clamped to 0-255
  4. Int16Array: An array of 16-bit signed integers
  5. Uint16Array: An array of 16-bit unsigned integers
  6. Int32Array: An array of 32-bit signed integers
  7. Uint32Array: An array of 32-bit unsigned integers
  8. BigInt64Array: An array of 64-bit signed BigInts
  9. BigUint64Array: An array of 64-bit unsigned BigInts
  10. Float16Array: An array of 16-bit floating point numbers
  11. Float32Array: An array of 32-bit floating point numbers
  12. Float64Array: An array of 64-bit floating point numbers

  For example, you can create an array of 8-bit signed integers as below

  ```javascript
  const a = new Int8Array();
  // You can pre-allocate n bytes
  const bytes = 1024;
  const a = new Int8Array(bytes);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The module loaders provides the below features,

  1. Dynamic loading
  2. State isolation
  3. Global namespace isolation
  4. Compilation hooks
  5. Nested virtualization

  **[⬆ Back to Top](#table-of-contents)**

  Collation is used for sorting a set of strings and searching within a set of strings. It is parameterized by locale and aware of Unicode. Let's take comparison and sorting features,

  1. **Comparison:**

  ```javascript
  var list = ["ä", "a", "z"]; // In German,  "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"
  var l10nDE = new Intl.Collator("de");
  var l10nSV = new Intl.Collator("sv");
  console.log(l10nDE.compare("ä", "z") === -1); // true
  console.log(l10nSV.compare("ä", "z") === +1); // true
  ```

  2. **Sorting:**

  ```javascript
  var list = ["ä", "a", "z"]; // In German,  "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"
  var l10nDE = new Intl.Collator("de");
  var l10nSV = new Intl.Collator("sv");
  console.log(list.sort(l10nDE.compare)); // [ "a", "ä", "z" ]
  console.log(list.sort(l10nSV.compare)); // [ "a", "z", "ä" ]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The for...of statement creates a loop iterating over iterable objects or elements such as built-in String, Array, Array-like objects (like arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. The basic usage of for...of statement on arrays would be as below,

  ```javascript
  let arrayIterable = [10, 20, 30, 40, 50];

  for (let value of arrayIterable) {
    value++;
    console.log(value); // 11 21 31 41 51
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  ```javascript
  [..."John Resig"];
  ```

  The output of the array is ['J', 'o', 'h', 'n', ' ', 'R', 'e', 's', 'i', 'g']

  **Explanation:** The string is an iterable type and the spread operator within an array maps every character of an iterable to one element. Hence, each character of a string becomes an element within an Array.

  **[⬆ Back to Top](#table-of-contents)**

  Yes, postMessages can be considered very secure as long as the programmer/developer is careful about checking the origin and source of an arriving message. But if you try to send/receive a message without verifying its source will create cross-site scripting attacks.

  **[⬆ Back to Top](#table-of-contents)**

  The second argument of postMessage method specifies which origin is allowed to receive the message. If you use the wildcard “\*” as an argument then any origin is allowed to receive the message. In this case, there is no way for the sender window to know if the target window is at the target origin when sending the message. If the target window has been navigated to another origin, the other origin would receive the data. Hence, this may lead to XSS vulnerabilities.

  ```javascript
  targetWindow.postMessage(message, "*");
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Since the listener listens for any message, an attacker can trick the application by sending a message from the attacker’s origin, which gives an impression that the receiver received the message from the actual sender’s window. You can avoid this issue by validating the origin of the message on the receiver's end using the “message.origin” attribute.

  For example, let's check the sender's origin [http://www.some-sender.com](http://www.some-sender.com) on receiver side [www.some-receiver.com](www.some-receiver.com),

  ```javascript
  //Listener on http://www.some-receiver.com/
  window.addEventListener("message", function(message){
      if(/^http://www\.some-sender\.com$/.test(message.origin)){
           console.log('You received the data from valid sender', message.data);
     }
  });
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You cannot avoid using postMessages completely(or 100%). Even though your application doesn’t use postMessage considering the risks, a lot of third party scripts use postMessage to communicate with the third party service. So your application might be using postMessage without your knowledge.

  **[⬆ Back to Top](#table-of-contents)**

  The postMessages are synchronous in IE8 browser but they are asynchronous in IE9 and all other modern browsers (i.e, IE9+, Firefox, Chrome, Safari).Due to this asynchronous behaviour, we use a callback mechanism when the postMessage is returned.

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript is a multi-paradigm language, supporting imperative/procedural programming, Object-Oriented Programming and functional programming. JavaScript supports Object-Oriented Programming with prototypical inheritance.

  **[⬆ Back to Top](#table-of-contents)**

  **Internal JavaScript:** It is the source code within the script tag.
  **External JavaScript:** The source code is stored in an external file(stored with .js extension) and referred with in the tag.

  **[⬆ Back to Top](#table-of-contents)**

  Yes, JavaScript is faster than server side scripts. Because JavaScript is a client-side script it does not require any web server’s help for its computation or calculation. So JavaScript is always faster than any server-side script like ASP, PHP, etc.

  **[⬆ Back to Top](#table-of-contents)**

  You can apply the `checked` property on the selected checkbox in the DOM. If the value is `true` it means the checkbox is checked, otherwise it is unchecked. For example, the below HTML checkbox element can be access using javascript as below:

  ```html
  <input type="checkbox" id="checkboxname" value="Agree" />
  Agree the conditions
  <br />
  ```

  ```javascript
  console.log(document.getElementById(‘checkboxname’).checked); // true or false
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The double tilde operator(~~) is known as double NOT bitwise operator. This operator is a slightly quicker substitute for Math.floor().

  **[⬆ Back to Top](#table-of-contents)**

  You can use the `String.prototype.charCodeAt()` method to convert string characters to ASCII numbers. For example, let's find ASCII code for the first letter of 'ABC' string,

  ```javascript
  "ABC".charCodeAt(0); // returns 65
  ```

  Whereas `String.fromCharCode()` method converts numbers to equal ASCII characters.

  ```javascript
  String.fromCharCode(65, 66, 67); // returns 'ABC'
  ```

  **[⬆ Back to Top](#table-of-contents)**

  An ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. You can create it as below,

  ```javascript
  let buffer = new ArrayBuffer(16); // create a buffer of length 16
  alert(buffer.byteLength); // 16
  ```

  To manipulate an ArrayBuffer, we need to use a “view” object.

  ```javascript
  //Create a DataView referring to the buffer
  let view = new DataView(buffer);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  ```javascript
  console.log("Welcome to JS world"[0]);
  ```

  The output of the above expression is "W".
  **Explanation:** The bracket notation with specific index on a string returns the character at a specific location. Hence, it returns the character "W" of the string. Since this is not supported in IE7 and below versions, you may need to use the .charAt() method to get the desired result.

  **[⬆ Back to Top](#table-of-contents)**

  The Error constructor creates an error object and the instances of error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions. The syntax of error object would be as below,

  ```javascript
  new Error([message[, fileName[, lineNumber]]])
  ```

  You can throw user defined exceptions or errors using Error object in try...catch block as below,

  ```javascript
  try {
    if (withdraw > balance)
      throw new Error("Oops! You don't have enough balance");
  } catch (e) {
    console.log(e.name + ": " + e.message);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The EvalError object indicates an error regarding the global `eval()` function. Even though this exception is not thrown by JavaScript anymore, the EvalError object remains for compatibility. The syntax of this expression would be as below,

  ```javascript
  new EvalError([message[, fileName[, lineNumber]]])
  ```

  You can throw EvalError with in try...catch block as below,

  ```javascript
  try {
    throw new EvalError('Eval function error', 'someFile.js', 100);
  } catch (e) {
    console.log(e.message, e.name, e.fileName);              // "Eval function error", "EvalError", "someFile.js"
  ```

  **[⬆ Back to Top](#table-of-contents)**

  When you apply 'use strict'; syntax, some of the below cases will throw a SyntaxError before executing the script

  1. When you use Octal syntax

  ```javascript
  var n = 022;
  ```

  2. Using `with` statement
  3. When you use delete operator on a variable name
  4. Using eval or arguments as variable or function argument name
  5. When you use newly reserved keywords
  6. When you declare a function in a block and access it from outside of the block

  ```javascript
  if (someCondition) {
    function f() {}
  }
  f(); // ReferenceError: f is not defined
  ```

  Hence, the errors from above cases are helpful to avoid errors in development/production environments.

  **[⬆ Back to Top](#table-of-contents)**

  No. All objects have prototypes except two exceptions:
  *  **Object.prototype** itself — This is the base object in the prototype chain, and **its prototype is** `null`.
  *   **Objects created with** `**Object.create(null)**` — These are deliberately created with **no prototype**, so they don’t inherit from `Object.prototype`.

  All other standard objects do have a prototype.  

  **[⬆ Back to Top](#table-of-contents)**

  Parameter is the variable name of a function definition whereas an argument represents the value given to a function when it is invoked. Let's explain this with a simple function

  ```javascript
  function myFunction(parameter1, parameter2, parameter3) {
    console.log(arguments[0]); // "argument1"
    console.log(arguments[1]); // "argument2"
    console.log(arguments[2]); // "argument3"
  }
  myFunction("argument1", "argument2", "argument3");
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The some() method is used to test whether at least one element in the array passes the test implemented by the provided function. The method returns a boolean value. Let's take an example to test for any odd elements,

  ```javascript
  var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

  var odd = (element) => element % 2 !== 0;

  console.log(array.some(odd)); // true (the odd element exists)
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The concat() method is used to join two or more arrays by returning a new array containing all the elements. The syntax would be as below,

  ```javascript
  array1.concat(array2, array3, ..., arrayX)
  ```

  Let's take an example of array's concatenation with veggies and fruits arrays,

  ```javascript
  var veggies = ["Tomato", "Carrot", "Cabbage"];
  var fruits = ["Apple", "Orange", "Pears"];
  var veggiesAndFruits = veggies.concat(fruits);
  console.log(veggiesAndFruits); // Tomato, Carrot, Cabbage, Apple, Orange, Pears
  ```

  **[⬆ Back to Top](#table-of-contents)**

  There are two ways to copy an object,

  **Shallow Copy:**
  Shallow copy is a bitwise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.

  **Example**

  ```javascript
  var empDetails = {
    name: "John",
    age: 25,
    expertise: "Software Developer",
  };
  ```

  to create a duplicate

  ```javascript
  var empDetailsShallowCopy = empDetails; //Shallow copying!
  ```

  if we change some property value in the duplicate one like this:

  ```javascript
  empDetailsShallowCopy.name = "Johnson";
  ```

  The above statement will also change the name of `empDetails`, since we have a shallow copy. That means we're losing the original data as well.

  **Deep copy:**
  A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.

  **Example**

  ```javascript
  var empDetails = {
    name: "John",
    age: 25,
    expertise: "Software Developer",
  };
  ```

  Create a deep copy by using the properties from the original object into new variable

  ```javascript
  var empDetailsDeepCopy = {
    name: empDetails.name,
    age: empDetails.age,
    expertise: empDetails.expertise,
  };
  ```

  Now if you change `empDetailsDeepCopy.name`, it will only affect `empDetailsDeepCopy` & not `empDetails`

  **[⬆ Back to Top](#table-of-contents)**

  The `repeat()` method is used to construct and return a new string which contains the specified number of copies of the string on which it was called, concatenated together. Remember that this method has been added to the ECMAScript 2015 specification.
  Let's take an example of Hello string to repeat it 4 times,

  ```javascript
  "Hello".repeat(4); // 'HelloHelloHelloHello'
  ```

  The `matchAll()` method can be used to return an iterator of all results matching a string against a regular expression. For example, the below example returns an array of matching string results against a regular expression,

  ```javascript
  let regexp = /Hello(\d?)/g;
  let greeting = "Hello1Hello2Hello3";

  let greetingList = [...greeting.matchAll(regexp)];

  console.log(greetingList[0][0]); //Hello1
  console.log(greetingList[1][0]); //Hello2
  console.log(greetingList[2][0]); //Hello3
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `trim` method of string prototype is used to trim on both sides of a string. But if you want to trim especially at the beginning or ending of the string then you can use `trimStart/trimLeft` and `trimEnd/trimRight` methods. Let's see an example of these methods on a greeting message,

  ```javascript
  var greeting = "   Hello, Goodmorning!   ";

  console.log(greeting); // "   Hello, Goodmorning!   "
  console.log(greeting.trimStart()); // "Hello, Goodmorning!   "
  console.log(greeting.trimLeft()); // "Hello, Goodmorning!   "

  console.log(greeting.trimEnd()); // "   Hello, Goodmorning!"
  console.log(greeting.trimRight()); // "   Hello, Goodmorning!"
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Let's take console statement with unary operator as given below,

  ```javascript
  console.log(+"Hello"); // NaN
  ```

  The output of the above console log statement returns NaN. Because the element is prefixed by the unary operator and the JavaScript interpreter will try to convert that element into a number type. Since the conversion fails, the value of the statement results in NaN value.

  **[⬆ Back to Top](#table-of-contents)**

  JavaScript does not have built-in support for mixins as a formal language feature. However, developers commonly implement mixins using various patterns to enable code reuse and composition.

  A mixin is a way to add reusable functionality from one or more objects into a class or another object, without using classical inheritance. It promotes object composition by combining behaviors or properties from different sources into a single destination.

  const canWalk = {
    walk() {
      console.log("Walking...");
    }
  };

  const canRead = {
    read() {
      console.log("Reading...");
    }
  };

  // Create a class
  class Person {
    constructor(name) {
      this.name = name;
    }
  }

  // Apply mixins
  Object.assign(Person.prototype, canEat, canWalk, canRead);

  // Use it
  const person = new Person("Sudheer");
  person.eat();  // Output: Eating...
  person.walk(); // Output: Walking...
  person.read(); // Output: Reading...
  ```

  Modern JavaScript favors mixin alternatives like composition, delegation, higher-order functions, and class mixins to promote reusable and modular code. Libraries like Lodash offer utilities for object composition, while frameworks like Vue.js provide built-in mixin features to promote reusable and modular code.

  **[⬆ Back to Top](#table-of-contents)**

  A thunk is just a function which delays the evaluation of the value. It doesn’t take any arguments but gives the value whenever you invoke the thunk. i.e, It is used not to execute now but it will be sometime in the future. Let's take a synchronous example,

  ```javascript
  const add = (x, y) => x + y;

  const thunk = () => add(2, 3);

  thunk(); // 5
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The asynchronous thunks are useful to make network requests. Let's see an example of network requests,

  ```javascript
  function fetchData(fn) {
    fetch("https://jsonplaceholder.typicode.com/todos/1")
      .then((response) => response.json())
      .then((json) => fn(json));
  }

  const asyncThunk = function () {
    return fetchData(function getData(data) {
      console.log(data);
    });
  };

  asyncThunk();
  ```

  The `getData` function won't be called immediately but it will be invoked only when the data is available from API endpoint. The setTimeout function is also used to make our code asynchronous. The best real time example is redux state management library which uses the asynchronous thunks to delay the actions to dispatch.

  **[⬆ Back to Top](#table-of-contents)**

  **Code snippet:**

  ```javascript
  const circle = {
    radius: 20,
    diameter() {
      return this.radius * 2;
    },
    perimeter: () => 2 * Math.PI * this?.radius,
  };
  ```

  ```javascript
  console.log(circle.diameter());
  console.log(circle.perimeter());
  ```

  **Output:**

  The output is 40 and NaN. Remember that diameter is a regular function, whereas the value of perimeter is an arrow function. The `this` keyword of a regular function(i.e, diameter) refers to the surrounding scope which is a class(i.e, Shape object). Whereas this keyword of perimeter function refers to the surrounding scope which is a window object. Since there is no radius property on window objects it returns an undefined value and the multiple of number value returns NaN value.

  **[⬆ Back to Top](#table-of-contents)**

  The easiest approach is using regular expressions to detect and replace newlines in the string. In this case, we use replace function along with string to replace with, which in our case is an empty string.

  ```javascript
  function remove_linebreaks( var message ) {
      return message.replace( /[\r\n]+/gm, "" );
  }
  ```

  In the above expression, g and m are for global and multiline flags.

  **[⬆ Back to Top](#table-of-contents)**

  A _repaint_ occurs when changes are made which affect the visibility of an element, but not its layout. Examples of this include outline, visibility, or background color. A _reflow_ involves changes that affect the layout of a portion of the page (or the whole page). Resizing the browser window, changing the font, content changing (such as user typing text), using JavaScript methods involving computed styles, adding or removing elements from the DOM, and changing an element's classes are a few of the things that can trigger reflow. Reflow of an element causes the subsequent reflow of all child and ancestor elements as well as any elements following it in the DOM.

  **[⬆ Back to Top](#table-of-contents)**

  Negating an array with `!` character will coerce the array into a boolean. Since Arrays are considered to be truthy So negating it will return `false`.

  ```javascript
  console.log(![]); // false
  ```

  **[⬆ Back to Top](#table-of-contents)**

  If you add two arrays together, it will convert them both to strings and concatenate them. For example, the result of adding arrays would be as below,

  ```javascript
  console.log(["a"] + ["b"]); // "ab"
  console.log([] + []); // ""
  console.log(![] + []); // "false", because ![] returns false.
  ```

  **[⬆ Back to Top](#table-of-contents)**

  If you prepend the additive(+) operator on falsy values(null, undefined, NaN, false, ""), the falsy value converts to a number value zero. Let's display them on browser console as below,

  ```javascript
  console.log(+null); // 0
  console.log(+undefined); // NaN
  console.log(+false); // 0
  console.log(+NaN); // NaN
  console.log(+""); // 0
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The self string can be formed with the combination of `[]()!+` characters. You need to remember the below conventions to achieve this pattern.

  1. Since Arrays are truthful values, negating the arrays will produce false: ![] === false
  2. As per JavaScript coercion rules, the addition of arrays together will toString them: [] + [] === ""
  3. Prepend an array with + operator will convert an array to false, the negation will make it true and finally converting the result will produce value '1': +(!(+[])) === 1

  By applying the above rules, we can derive below conditions

  ```javascript
  (![] + [] === "false" + !+[]) === 1;
  ```

  Now the character pattern would be created as below,

  ```javascript
        s               e               l               f
   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^

   (![] + [])[3] + (![] + [])[4] + (![] + [])[2] + (![] + [])[0]
   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^
  (![] + [])[+!+[]+!+[]+!+[]] +
  (![] + [])[+!+[]+!+[]+!+[]+!+[]] +
  (![] + [])[+!+[]+!+[]] +
  (![] + [])[+[]]
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  (![]+[])[+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]]+(![]+[])[+[]]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can apply the filter method on the array by passing Boolean as a parameter. This way it removes all falsy values(0, undefined, null, false and "") from the array.

  ```javascript
  const myArray = [false, null, 1, 5, undefined];
  myArray.filter(Boolean); // [1, 5] // is same as myArray.filter(x => x);
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can get unique values of an array with the combination of `Set` and rest expression/spread(...) syntax.

  ```javascript
  console.log([...new Set([1, 2, 4, 4, 3])]); // [1, 2, 4, 3]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Sometimes you would like to have a destructured variable with a different name than the property name. In that case, you'll use a `: newName` to specify a name for the variable. This process is called destructuring aliases.

  ```javascript
  const obj = { x: 1 };
  // Grabs obj.x as { otherName }
  const { x: otherName } = obj;
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can map the array values without using the `map` method by just using the `from` method of Array. Let's map city names from Countries array,

  ```javascript
  const countries = [
    { name: "India", capital: "Delhi" },
    { name: "US", capital: "Washington" },
    { name: "Russia", capital: "Moscow" },
    { name: "Singapore", capital: "Singapore" },
    { name: "China", capital: "Beijing" },
    { name: "France", capital: "Paris" },
  ];

  const cityNames = Array.from(countries, ({ capital }) => capital);
  console.log(cityNames); // ['Delhi, 'Washington', 'Moscow', 'Singapore', 'Beijing', 'Paris']
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can empty an array quickly by setting the array length to zero.

  ```javascript
  let cities = ["Singapore", "Delhi", "London"];
  cities.length = 0; // cities becomes []
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can round numbers to a certain number of decimals using `toFixed` method from native javascript.

  ```javascript
  let pie = 3.141592653;
  pie = pie.toFixed(3); // 3.142
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can convert an array to an object with the same data using spread(...) operator.

  ```javascript
  var fruits = ["banana", "apple", "orange", "watermelon"];
  var fruitsObject = { ...fruits };
  console.log(fruitsObject); // {0: "banana", 1: "apple", 2: "orange", 3: "watermelon"}
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can create an array with some data or an array with the same values using `fill` method.

  ```javascript
  var newArray = new Array(5).fill("0");
  console.log(newArray); // ["0", "0", "0", "0", "0"]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Below are the list of placeholders available from console object,

  1. %o — It takes an object,
  2. %s — It takes a string,
  3. %d — It is used for a decimal or integer
     These placeholders can be represented in the console.log as below

  ```javascript
  const user = { name: "John", id: 1, city: "Delhi" };
  console.log(
    "Hello %s, your details %o are available in the object form",
    "John",
    user
  ); // Hello John, your details {name: "John", id: 1, city: "Delhi"} are available in object
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Yes, you can apply CSS styles to console messages similar to html text on the web page.

  ```javascript
  console.log(
    "%c The text has blue color, with large font and red background",
    "color: blue; font-size: x-large; background: red"
  );
  ```

  The text will be displayed as below,
  ![Screenshot](images/console-css.png)

  **Note:** All CSS styles can be applied to console messages.

  **[⬆ Back to Top](#table-of-contents)**

  The `console.dir()` is used to display an interactive list of the properties of the specified JavaScript object as JSON.

  ```javascript
  const user = { name: "John", id: 1, city: "Delhi" };
  console.dir(user);
  ```

  The user object displayed in JSON representation
  ![Screenshot](images/console-dir.png)

  **[⬆ Back to Top](#table-of-contents)**

  Yes, it is possible to get and debug HTML elements in the console just like inspecting elements.

  ```javascript
  const element = document.getElementsByTagName("body")[0];
  console.log(element);
  ```

  It prints the HTML element in the console,

  ![Screenshot](images/console-html.png)

  **[⬆ Back to Top](#table-of-contents)**

  The `console.table()` is used to display data in the console in a tabular format to visualize complex arrays or objects.

  ```js
  const users = [
    { name: "John", id: 1, city: "Delhi" },
    { name: "Max", id: 2, city: "London" },
    { name: "Rod", id: 3, city: "Paris" },
  ];
  console.table(users);
  ```

  The data visualized in a table format,

  ![Screenshot](images/console-table.png)
  **Not:** Remember that `console.table()` is not supported in IE.

  **[⬆ Back to Top](#table-of-contents)**

  The combination of IsNaN and isFinite methods are used to confirm whether an argument is a number or not.

  ```javascript
  function isNumber(n) {
    return !isNaN(parseFloat(n)) && isFinite(n);
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You need to select the content(using .select() method) of the input element and execute the copy command with execCommand (i.e, execCommand('copy')). You can also execute other system commands like cut and paste.

  ```javascript
  document.querySelector("#copy-button").onclick = function () {
    // Select the content
    document.querySelector("#copy-input").select();
    // Copy to the clipboard
    document.execCommand("copy");
  };
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use `new Date().getTime()` to get the current timestamp. There is an alternative shortcut to get the value.

  ```javascript
  console.log(+new Date());
  console.log(Date.now());
  ```

  **[⬆ Back to Top](#table-of-contents)**

  Flattening bi-dimensional arrays is trivial with Spread operator.

  ```javascript
  const biDimensionalArr = [11, [22, 33], [44, 55], [66, 77], 88, 99];
  const flattenArr = [].concat(...biDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
  ```

  But you can make it work with multi-dimensional arrays by recursive calls,

  ```javascript
  function flattenMultiArray(arr) {
    const flattened = [].concat(...arr);
    return flattened.some((item) => Array.isArray(item))
      ? flattenMultiArray(flattened)
      : flattened;
  }
  const multiDimensionalArr = [
    11,
    [22, 33],
    [44, [55, 66, [77, [88]], 99]],
  ];
  const flatArr = flattenMultiArray(multiDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
  ```

  Also you can use the `flat` method of Array.

  ```javascript
  const arr = [1, [2, 3], 4, 5, [6, 7]];
  const fllattenArr = arr.flat(); // [1, 2, 3, 4, 5, 6, 7]

  // And for multiDimensional arrays
  const multiDimensionalArr = [
    11,
    [22, 33],
    [44, [55, 66, [77, [88]], 99]],
  ];
  const oneStepFlat = multiDimensionalArr.flat(1); // [11, 22, 33, 44, [55, 66, [77, [88]], 99]]
  const towStep = multiDimensionalArr.flat(2); // [11, 22, 33, 44, 55, 66, [77, [88]], 99]
  const fullyFlatArray = multiDimensionalArr.flat(Infinity); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
  ```

  **[⬆ Back to Top](#table-of-contents)**

  You can use `indexOf` to compare input with multiple values instead of checking each value as one condition.

  ```javascript
  // Verbose approach
  if (
    input === "first" ||
    input === 1 ||
    input === "second" ||
    input === 2
  ) {
    someFunction();
  }
  // Shortcut
  if (["first", 1, "second", 2].indexOf(input) !== -1) {
    someFunction();
  }
  ```

  **[⬆ Back to Top](#table-of-contents)**

  The `beforeunload` event is triggered when the window, the document and its resources are about to be unloaded. This event is helpful to warn users about losing the current data and detect back button event.

  ```javascript
  window.addEventListener("beforeunload", () => {
    console.log("Clicked browser back button");
  });
  ```

  You can also use `popstate` event to detect the browser back button.
  **Note:** The history entry has been activated using `history.pushState` method.

  ```javascript
  window.addEventListener("popstate", () => {
    console.log("Clicked browser back button");
    box.style.backgroundColor = "white";
  });

  const box = document.getElementById("div");

  box.addEventListener("click", () => {
    box.style.backgroundColor = "blue";
    window.history.pushState({}, null, null);
  });
  ```


In the preceeding code, When the box element clicked, its background color appears in blue color and changed to while color upon clicking the browser back button using `popstate` event handler. The `state` property of `popstate` contains the copy of history entry's state object.

 **[⬆ Back to Top](#table-of-contents)**

 The right click on the page can be disabled by returning false from the `oncontextmenu` attribute on the body element.

 ```html
 <body oncontextmenu="return false;"></body>
 ```

 **[⬆ Back to Top](#table-of-contents)**

 Primitive Values like string,number and boolean don't have properties and methods but they are temporarily converted or coerced to an object(Wrapper object) when you try to perform actions on them. For example, if you apply toUpperCase() method on a primitive string value, it does not throw an error but returns uppercase of the string.

 ```javascript
 let name = "john";

 console.log(name.toUpperCase()); // Behind the scenes treated as console.log(new String(name).toUpperCase());
 ```

 i.e, Every primitive except null and undefined have Wrapper Objects and the list of wrapper objects are String,Number,Boolean,Symbol and BigInt.

 **[⬆ Back to Top](#table-of-contents)**

 AJAX stands for Asynchronous JavaScript and XML and it is a group of related technologies(HTML, CSS, JavaScript, XMLHttpRequest API etc) used to display data asynchronously. i.e. We can send data to the server and get data from the server without reloading the web page.

 **[⬆ Back to Top](#table-of-contents)**

 Below are the list of different ways to deal with Asynchronous code.

 1. Callbacks
 2. Promises
 3. Async/await
 4. Third-party libraries such as async.js,bluebird etc

 **[⬆ Back to Top](#table-of-contents)**

 Until a few days back, One shortcoming of native promises is no direct way to cancel a fetch request. But the new `AbortController` from js specification allows you to use a signal to abort one or multiple fetch calls.
 The basic flow of cancelling a fetch request would be as below,

 1. Create an `AbortController` instance
 2. Get the signal property of an instance and pass the signal as a fetch option for signal
 3. Call the AbortController's abort property to cancel all fetches that use that signal
    For example, passing the same signal to multiple fetch calls will cancel all requests with that signal,

 ```javascript
 const controller = new AbortController();
 const { signal } = controller;

 fetch("http://localhost:8000", { signal })
   .then((response) => {
     console.log(`Request 1 is complete!`);
   })
   .catch((e) => {
     if (e.name === "AbortError") {
       // We know it's been canceled!
     }
   });

 fetch("http://localhost:8000", { signal })
   .then((response) => {
     console.log(`Request 2 is complete!`);
   })
   .catch((e) => {
     if (e.name === "AbortError") {
       // We know it's been canceled!
     }
   });

 // Wait 2 seconds to abort both requests
 setTimeout(() => controller.abort(), 2000);
 ```

 **[⬆ Back to Top](#table-of-contents)**

 Web speech API is used to enable modern browsers recognize and synthesize speech(i.e, voice data into web apps). This API was introduced by W3C Community in the year 2012. It has two main parts:

 1. **SpeechRecognition (Asynchronous Speech Recognition or Speech-to-Text):** It provides the ability to recognize voice context from an audio input and respond accordingly. This is accessed by the `SpeechRecognition` interface.
    The example below shows how to use this API to get text from speech,

 ```javascript
 window.SpeechRecognition =
   window.webkitSpeechRecognition || window.SpeechRecognition; // webkitSpeechRecognition for Chrome and SpeechRecognition for FF
 const recognition = new window.SpeechRecognition();
 recognition.onresult = (event) => {
   // SpeechRecognitionEvent type
   const speechToText = event.results[0][0].transcript;
   console.log(speechToText);
 };
 recognition.start();
 ```

 In this API, browser is going to ask you for permission to use your microphone

 2. **SpeechSynthesis (Text-to-Speech):** It provides the ability to recognize voice context from an audio input and respond. This is accessed by the `SpeechSynthesis` interface.
    For example, the below code is used to get voice/speech from text,

 ```javascript
 if ("speechSynthesis" in window) {
   var speech = new SpeechSynthesisUtterance("Hello World!");
   speech.lang = "en-US";
   window.speechSynthesis.speak(speech);
 }
 ```

 The above examples can be tested on chrome(33+) browser's developer console.
 **Note:** This API is still a working draft and only available in Chrome and Firefox browsers(ofcourse Chrome only implemented the specification)

 **[⬆ Back to Top](#table-of-contents)**

 Both browser and NodeJS javascript environments throttles with a minimum delay that is greater than 0ms. That means even though setting a delay of 0ms will not happen instantaneously.
 **Browsers:** They have a minimum delay of 4ms. This throttle occurs when successive calls are triggered due to callback nesting(certain depth) or after a certain number of successive intervals.
 Note: The older browsers have a minimum delay of 10ms.
 **Nodejs:** They have a minimum delay of 1ms. This throttle happens when the delay is larger than 2147483647 or less than 1.
 The best example to explain this timeout throttling behavior is the order of below code snippet.

 ```javascript
 function runMeFirst() {
   console.log("My script is initialized");
 }
 setTimeout(runMeFirst, 0);
 console.log("Script loaded");
 ```

 and the output would be in

 ```cmd
 Script loaded
 My script is initialized
 ```

 If you don't use `setTimeout`, the order of logs will be sequential.

 ```javascript
 function runMeFirst() {
   console.log("My script is initialized");
 }
 runMeFirst();
 console.log("Script loaded");
 ```

 and the output is,

 ```cmd
 My script is initialized
 Script loaded
 ```

 **[⬆ Back to Top](#table-of-contents)**

 You can't use setTimeout(fn, 0) to execute the code immediately due to minimum delay of greater than 0ms. But you can use window.postMessage() to achieve this behavior.

 **[⬆ Back to Top](#table-of-contents)**

 A task is any javascript code/program which is scheduled to be run by the standard mechanisms such as initially starting to run a program, run an event callback, or an interval or timeout being fired. All these tasks are scheduled on a task queue.
 Below are the list of use cases to add tasks to the task queue,

 1. When a new javascript program is executed directly from console or running by the `<script>` element, the task will be added to the task queue.
 2. When an event fires, the event callback added to task queue
 3. When a setTimeout or setInterval is reached, the corresponding callback added to task queue

 **[⬆ Back to Top](#table-of-contents)**

 A microtask is a type of JavaScript callback that is scheduled to run immediately after the currently executing script and before the next event loop tick. Microtasks are executed after the current task completes and before any new tasks (macrotasks) are run. This ensures a fast and predictable update cycle.

 Common sources of microtasks stored in the microtask queue include:

 1. **Promises:**
    When a Promise is resolved or rejected, its `.then()`, `.catch()`, and `.finally()` callbacks are placed in the microtask queue.

    ```javascript
    Promise.resolve().then(() => {
     console.log('Microtask from a Promise');
    });
    ```

  2. **queueMicrotask():**
 
     A method that explicitly schedules a function to be run in the microtask queue.

     ```javascript
      queueMicrotask(() => {
         console.log('Microtask from  queueMicrotask');
       });
     ```

  3. **MutationObserver callbacks:**

     Observers changes in the DOM and triggers a callback as a microtask.

     ```javascript
        const observer = new MutationObserver(() => {
          console.log('Microtask from MutationObserver');
        })
        observer.observe(document.body, {childList: true});
     ```

  4. **await:**  
     Await internally uses Promises, so the code after `await` is scheduled as a microtask.

     ```javascript
      async function asyncFunction() {
        await null;
        console.log('Microtask from Await'); // Schedule this code as microtask
      }
     ```
 **Note:** All of these microtasks are processed in the same turn of the event loop.

 **[⬆ Back to Top](#table-of-contents)**

 In JavaScript, there are multiple event loops that can be used depending on the context of your application. The most common event loops are:

 1. The Browser Event Loop
 2. The Node.js Event Loop

Debouncing is a programming technique used to limit how often a function is executed. Specifically, it ensures that a function is only triggered after a certain amount of time has passed since it was last invoked. This prevents unnecessary or excessive function calls, which can help optimize performance and reduce unnecessary CPU usage or API requests.

For example, when a user types in a search box, you typically want to wait until they’ve finished typing before fetching suggestions. Without debouncing, an API call would be triggered on every keystroke, potentially causing performance issues. With debouncing, the function call is postponed until the user stops typing for a specified period (e.g., 300ms). If the user types again before this time elapses, the timer resets.

**Typical use cases for debouncing include:**

*   Search box suggestions (wait until typing pauses before fetching results)
*   Auto-saving text fields (save only after the user stops typing)
*   Preventing double-clicks on buttons
*   Handling window resize or scroll events efficiently

**Example Debounce Function:**

JavaScript

```css
function debounce(func, timeout = 500) {
  let timer;
  return function (...args) {
    clearTimeout(timer);
    timer = setTimeout(() => {
      func.apply(this, args);
    }, timeout);
  };
}
```

**Usage Example:**

JavaScript

```css
function fetchResults() {
  console.log("Fetching input suggestions");
}
const processChange = debounce(fetchResults, 300);

// Attach to input element
<input type="text" onkeyup="processChange()" />

// Attach to button
<button onclick="processChange()">Click me</button>

// Attach to window event
window.addEventListener("scroll", processChange);
```

**How it works:**  
When `processChange` is invoked (e.g., by typing or clicking), any pending execution is canceled, and the function is scheduled to run after the specified delay. If another event occurs before the delay is up, the timer resets, and the function will only run after events have stopped for the delay duration.

Debouncing is an essential tool for improving user experience and application performance, especially when dealing with events that can fire rapidly and repeatedly.

const add = (x, y) => x + y;
const multiplyBy2 = (x) => x * 2;

//Now add (2, 3) can be replaced by 5.

multiplyBy2(add(2, 3));

console.log(this);

function displayThis() {
  console.log(this);
}

displayThis();

const person = {
  name: "John",
  greet: function () {
    console.log("Hello, " + this.name);
  },
};

person.greet();

 document.getElementById("myButton").addEventListener("click", function () {
          console.log(this);
 });

      const obj = {
        age: 42,
        regular: function() { console.log(this.age); },
        arrow: () => { console.log(this.age); }
      };
      obj.regular(); // 42 (this refers to obj)
      obj.arrow();   // undefined (this refers to the outer scope, not obj)

  function Person(name) {
    this.name = name;
  }
  
  const p1 = new Person('Sudheer');
  console.log(p1.name); // Sudheer

Both map and forEach functions are used to iterate over an arrays but there are some differences in their functionality.

1. **Returning values:** The `map` method returns a new array with transformed elements whereas `forEach` method returns `undefined` even though both of them are doing the same job.

```javascript
  const arr = [1, 2, 3, 4, 5];
  arr.map(x => x * x); // [1, 4, 9, 16, 25]
  arr.forEach(x => x * x); //

  The `forEach()` method in JavaScript always returns undefined. This is because forEach() is used to iterate over arrays and perform side effects on each element, rather than returning a `new array or transforming the original array`
```

2. **Chaining methods:** The `map` method is chainable. i.e, It can be attached with `reduce`, `filter`, `sort` and other methods as well. Whereas `forEach` cannot be attached with any other methods because it returns `undefined` value.

```javascript
const arr = [1, 2, 3, 4, 5];
arr.map((x) => x * x).reduce((total, cur) => total + cur); // 55
arr.forEach((x) => x * x).reduce((total, cur) => total + cur); //Uncaught TypeError: Cannot read properties of undefine(reading 'reduce')
```

3. **Mutation:** The `map` method doesn't mutate the original array by returning new array. Whereas `forEach` method also doesn't mutate the original array but it's callback is allowed to mutate the original array.

**Note:** Both these methods existed since ES5 onwards.

let shape = { width: 30, height: 20 }; // Compiler store the type in cache as { width: <int>, height: <int>} after repeated calls

function area(obj) {
  //Calculate area
}
for (let i = 0; i < 100; i++) {
  area(shape);
}

const element = document.getElementById("myElement");
function animate() {
  let currentPosition = parseInt(window.getComputedStyle(element).left, 10);

  // Move the element 2px per frame
  currentPosition += 2;
  element.style.left = currentPosition + "px";
  // If the element hasn't moved off-screen, request the next frame
  if (currentPosition < window.innerWidth) {
    requestAnimationFrame(animate);
  }
}
// Start the animation
requestAnimationFrame(animate);

function multiply(x, y) {
  return x * y;
}

function sum(a, b, c) {
  return a + b + c;
}

console.log(multiply.length); //2
console.log(sum.length); //3

//1. browser environment
console.log(globalThis); // => Window {...}

//2. node.js environment
console.log(globalThis); // => Object [global] {...}

//3. web worker environment
console.log(globalThis); // => DedicatedWorkerGlobalScope {...}

// moduleA.js

// This variable is PRIVATE to moduleA. It's like a tool inside a closed box.
const privateVariable = "I am private";

// This variable is PUBLIC because it's exported. Others can use it when they import moduleA.
export const publicVariable = "I am public";

// PUBLIC function because it's exported. But it can still access privateVariable inside moduleA.
export function publicFunction() {
  console.log(privateVariable); // ✅ This works because we're inside the same module.
  return "Hello from publicFunction!";
}

// moduleB.js

// Importing PUBLIC items from moduleA.
import { publicVariable, publicFunction } from "./moduleA.js";

console.log(publicVariable); // ✅ "I am public" - Works because it's exported.
console.log(publicFunction()); // ✅ "Hello from publicFunction!" - Works as well.

// ❌ This will cause an ERROR because privateVariable was NOT exported from moduleA.
// console.log(privateVariable);   // ❌ ReferenceError: privateVariable is not defined

   const button = document.getElementById("btn");

    function handleClick() {
      console.log("Clicked!");
    }

    button.addEventListener("click", handleClick);

    // Always remove when done
    button.removeEventListener("click", handleClick);

const deepCopy = structuredClone(originalObject);

// Modify cloned value
deepCopy.nested.value = 99;
deepCopy.nested.list.push(4);
console.log(originalObject.nested.value); // 10
console.log(deepCopy.nested.value);       // 99
console.log(originalObject.nested.list);  // [1, 2, 3]
console.log(deepCopy.nested.list);        // [1, 2, 3, 4]