sveltejs/svelte-preprocess

Svelte Preprocess

A Svelte preprocessor with sensible defaults and support for: PostCSS, SCSS, Less, Stylus, CoffeeScript, TypeScript, Pug and much more.

• What is it?
• Getting Started
• Usage
• Migration Guide
• Preprocessing
  • Preprocessors
• Features
  • Template tag
  • External files
  • Global style
  • Modern JavaScript syntax
  • Replace values
  • Built-in support for commonly used languages

What is it?

Svelte's own parser understands only JavaScript, CSS and its HTML-like syntax. To make it possible to write components in other languages, such as TypeScript or SCSS, Svelte provides the preprocess API, which allows to easily transform the content of your markup and your style/script tags.

Writing your own preprocessor for, i.e SCSS is easy enough, but it can be cumbersome to have to always configure multiple preprocessors for the languages you'll be using.

svelte-preprocess is a custom svelte preprocessor that acts as a facilitator to use other languages with Svelte, providing multiple features, sensible defaults and a less noisy development experience.

It is recommended to use with svelte.config.js file, located at the project root. For other usage, please refer to usage documentation.

import { sveltePreprocess } from 'svelte-preprocess';

const config = {
  preprocess: sveltePreprocess({ ... })
}

export default config;

When to use it?

Historically, svelte-preprocess was the go-to option whenever you had a language other than JS/HTML/CSS used inside your Svelte files. Today, depending on your setup, you may no longer need it:

• If you're using Svelte 5 and are only using type features of TypeScript (i.e. things that just "go away" after transpiling to JS), you don't need any preprocessor, Svelte handles this natively
• If you're using Vite and are using TypeScript or common CSS preprocessors (PostCSS, SCSS, Less, ...), use the preprocessor that comes with vite-plugin-svelte instead

If these cases don't apply to you, use svelte-preprocess.

Features

Template tag

Vue-like support for defining your markup between a specific tag. The default tag is template but it can be customized.

<template>
  <div>Hey</div>
</template>

<style></style>

<script></script>

External files

<template src="./template.html"></template>
<script src="./script.js"></script>
<style src="./style.css"></style>

Note: using a relative path starting with . is important. Otherwise svelte-preprocess will ignore the src attribute.

Global style

global attribute

Add a global attribute to your style tag and instead of scoping the CSS, all of its content will be interpreted as global style.

<style global>
  div {
    color: red;
  }
</style>

:global rule

Use a :global rule to only expose parts of the stylesheet:

<style lang="scss">
  .scoped-style {
  }

  :global {
    @import 'global-stylesheet.scss';

    .global-style {
      .global-child-style {
      }
    }
  }
</style>

Works best with nesting-enabled CSS preprocessors, but regular CSS selectors like div :global .global1 .global2 are also supported.

Note: needs PostCSS to be installed.

Modern JavaScript syntax

svelte-preprocess allows you to run your component code through Babel before sending it to the compiler, allowing you to use new language features such as optional operators and nullish coalescing. However, note that Babel should transpile your component code to the javascript version supported by the Svelte compiler, so ES6+.

For example, with @babel/preset-env your config could be:

import { sveltePreprocess } from 'svelte-preprocess'
  ...
  preprocess: sveltePreprocess({
    babel: {
      presets: [
        [
          '@babel/preset-env',
          {
            loose: true,
            // No need for babel to resolve modules
            modules: false,
            targets: {
              // ! Very important. Target es6+
              esmodules: true,
            },
          },
        ],
      ],
    },
  });
  ...

Note: If you want to transpile your app to be supported in older browsers, you must run babel from the context of your bundler.

Replace values

Replace a set of string patterns in your components markup by passing an array of [RegExp, ReplaceFn | string], the same arguments received by the String.prototype.replace method.

In example, to replace inject the value of process.env.NODE_ENV:

autoPreprocess({
  replace: [[/process\.env\.NODE_ENV/g, JSON.stringify(process.env.NODE_ENV)]],
});

Which, in a production environment, would turn

{#if process.env.NODE_ENV !== 'development'}
  <h1>Production environment!</h1>
{/if}

into

{#if 'production' !== 'development'}
  <h1>Production environment!</h1>
{/if}

Built-in support for commonly used languages

The current supported languages out-of-the-box are Sass, Stylus, Less, CoffeeScript, TypeScript, Pug, PostCSS, Babel.

<template lang="pug">
  div Posts +each('posts as post') a(href="{post.url}") {post.title}
</template>

<script lang="ts">
  export const hello: string = 'world';
</script>

<style src="./style.scss"></style>

Getting started

Preprocessing documentation

Usage documentation

Migration Guide

License

MIT