max-mapper/mississippi

mississippi

a collection of useful stream utility modules. learn how the modules work using this and then pick the ones you want and use them individually

the goal of the modules included in mississippi is to make working with streams easy without sacrificing speed, error handling or composability.

usage

var miss = require('mississippi')

methods

• pipe
• each
• pipeline
• duplex
• through
• from
• to
• concat
• finished
• parallel

pipe

miss.pipe(stream1, stream2, stream3, ..., cb)

Pipes streams together and destroys all of them if one of them closes. Calls cb with (error) if there was an error in any of the streams.

When using standard source.pipe(destination) the source will not be destroyed if the destination emits close or error. You are also not able to provide a callback to tell when the pipe has finished.

miss.pipe does these two things for you, ensuring you handle stream errors 100% of the time (unhandled errors are probably the most common bug in most node streams code)

original module

miss.pipe is provided by require('pump')

example

// lets do a simple file copy
var fs = require('fs')

var read = fs.createReadStream('./original.zip')
var write = fs.createWriteStream('./copy.zip')

// use miss.pipe instead of read.pipe(write)
miss.pipe(read, write, function (err) {
  if (err) return console.error('Copy error!', err)
  console.log('Copied successfully')
})

each

miss.each(stream, each, [done])

Iterate the data in stream one chunk at a time. Your each function will be called with (data, next) where data is a data chunk and next is a callback. Call next when you are ready to consume the next chunk.

Optionally you can call next with an error to destroy the stream. You can also pass the optional third argument, done, which is a function that will be called with (err) when the stream ends. The err argument will be populated with an error if the stream emitted an error.

original module

miss.each is provided by require('stream-each')

example

var fs = require('fs')
var split = require('binary-split') // require('split2') would work here as well

var newLineSeparatedNumbers = fs.createReadStream('numbers.txt')

var pipeline = miss.pipeline(newLineSeparatedNumbers, split())
miss.each(pipeline, eachLine, done)
var sum = 0

function eachLine (line, next) {
  sum += parseInt(line.toString())
  next()
}

function done (err) {
  if (err) throw err
  console.log('sum is', sum)
}

pipeline

var pipeline = miss.pipeline(stream1, stream2, stream3, ...)

Builds a pipeline from all the transform streams passed in as arguments by piping them together and returning a single stream object that lets you write to the first stream and read from the last stream.

If you are pumping object streams together use pipeline = miss.pipeline.obj(s1, s2, ...).

If any of the streams in the pipeline emits an error or gets destroyed, or you destroy the stream it returns, all of the streams will be destroyed and cleaned up for you.

original module

miss.pipeline is provided by require('pumpify')

example

// first create some transform streams (note: these two modules are fictional)
var imageResize = require('image-resizer-stream')({width: 400})
var pngOptimizer = require('png-optimizer-stream')({quality: 60})

// instead of doing a.pipe(b), use pipeline
var resizeAndOptimize = miss.pipeline(imageResize, pngOptimizer)
// `resizeAndOptimize` is a transform stream. when you write to it, it writes
// to `imageResize`. when you read from it, it reads from `pngOptimizer`.
// it handles piping all the streams together for you

// use it like any other transform stream
var fs = require('fs')

var read = fs.createReadStream('./image.png')
var write = fs.createWriteStream('./resized-and-optimized.png')

miss.pipe(read, resizeAndOptimize, write, function (err) {
  if (err) return console.error('Image processing error!', err)
  console.log('Image processed successfully')
})

duplex

var duplex = miss.duplex([writable, readable, opts])

Take two separate streams, a writable and a readable, and turn them into a single duplex (readable and writable) stream.

The returned stream will emit data from the readable. When you write to it it writes to the writable.

You can either choose to supply the writable and the readable at the time you create the stream, or you can do it later using the .setWritable and .setReadable methods and data written to the stream in the meantime will be buffered for you.

original module

miss.duplex is provided by require('duplexify')

example

// lets spawn a process and take its stdout and stdin and combine them into 1 stream
var child = require('child_process')

// @- tells it to read from stdin, --data-binary sets 'raw' binary mode
var curl = child.spawn('curl -X POST --data-binary @- http://foo.com')

// duplexCurl will write to stdin and read from stdout
var duplexCurl = miss.duplex(curl.stdin, curl.stdout)

through

var transformer = miss.through([options, transformFunction, flushFunction])

Make a custom transform stream.

The options object is passed to the internal transform stream and can be used to create an objectMode stream (or use the shortcut miss.through.obj([...]))

The transformFunction is called when data is available for the writable side and has the signature (chunk, encoding, cb). Within the function, add data to the readable side any number of times with this.push(data). Call cb() to indicate processing of the chunk is complete. Or to easily emit a single error or chunk, call cb(err, chunk)

The flushFunction, with signature (cb), is called just before the stream is complete and should be used to wrap up stream processing.

original module

miss.through is provided by require('through2')

example

var fs = require('fs')

var read = fs.createReadStream('./boring_lowercase.txt')
var write = fs.createWriteStream('./AWESOMECASE.TXT')

// Leaving out the options object
var uppercaser = miss.through(
  function (chunk, enc, cb) {
    cb(null, chunk.toString().toUpperCase())
  },
  function (cb) {
    cb(null, 'ONE LAST BIT OF UPPERCASE')
  }
)

miss.pipe(read, uppercaser, write, function (err) {
  if (err) return console.error('Trouble uppercasing!')
  console.log('Splendid uppercasing!')
})

from

miss.from([opts], read)

Make a custom readable stream.

opts contains the options to pass on to the ReadableStream constructor e.g. for creating a readable object stream (or use the shortcut miss.from.obj([...])).

Returns a readable stream that calls read(size, next) when data is requested from the stream.

• size is the recommended amount of data (in bytes) to retrieve.
• next(err, chunk) should be called when you're ready to emit more data.

original module

miss.from is provided by require('from2')

example

function fromString(string) {
  return miss.from(function(size, next) {
    // if there's no more content
    // left in the string, close the stream.
    if (string.length <= 0) return next(null, null)

    // Pull in a new chunk of text,
    // removing it from the string.
    var chunk = string.slice(0, size)
    string = string.slice(size)

    // Emit "chunk" from the stream.
    next(null, chunk)
  })
}

// pipe "hello world" out
// to stdout.
fromString('hello world').pipe(process.stdout)

to

miss.to([options], write, [flush])

Make a custom writable stream.

opts contains the options to pass on to the WritableStream constructor e.g. for creating a writable object stream (or use the shortcut miss.to.obj([...])).

Returns a writable stream that calls write(data, enc, cb) when data is written to the stream.

• data is the received data to write the destination.
• enc encoding of the piece of data received.
• cb(err, data) should be called when you're ready to write more data, or encountered an error.

flush(cb) is called before finish is emitted and allows for cleanup steps to occur.

original module

miss.to is provided by require('flush-write-stream')

example

var ws = miss.to(write, flush)

ws.on('finish', function () {
  console.log('finished')
})

ws.write('hello')
ws.write('world')
ws.end()

function write (data, enc, cb) {
  // i am your normal ._write method
  console.log('writing', data.toString())
  cb()
}

function flush (cb) {
  // i am called before finish is emitted
  setTimeout(cb, 1000) // wait 1 sec
}

If you run the above it will produce the following output

writing hello
writing world
(nothing happens for 1 sec)
finished

concat

var concat = miss.concat(cb)

Returns a writable stream that concatenates all data written to the stream and calls a callback with the single result.

Calling miss.concat(cb) returns a writable stream. cb is called when the writable stream is finished, e.g. when all data is done being written to it. cb is called with a single argument, (data), which will contain the result of concatenating all the data written to the stream.

Note that miss.concat will not handle stream errors for you. To handle errors, use miss.pipe or handle the error event manually.

original module

miss.concat is provided by require('concat-stream')

example

var fs = require('fs')

var readStream = fs.createReadStream('cat.png')
var concatStream = miss.concat(gotPicture)

function callback (err) {
  if (err) {
    console.error(err)
    process.exit(1)
  }
}

miss.pipe(readStream, concatStream, callback)

function gotPicture(imageBuffer) {
  // imageBuffer is all of `cat.png` as a node.js Buffer
}

function handleError(err) {
  // handle your error appropriately here, e.g.:
  console.error(err) // print the error to STDERR
  process.exit(1) // exit program with non-zero exit code
}

finished

miss.finished(stream, cb)

Waits for stream to finish or error and then calls cb with (err). cb will only be called once. err will be null if the stream finished without error, or else it will be populated with the error from the streams error event.

This function is useful for simplifying stream handling code as it lets you handle success or error conditions in a single code path. It's used internally miss.pipe.

original module

miss.finished is provided by require('end-of-stream')

example

var copySource = fs.createReadStream('./movie.mp4')
var copyDest = fs.createWriteStream('./movie-copy.mp4')

copySource.pipe(copyDest)

miss.finished(copyDest, function(err) {
  if (err) return console.log('write failed', err)
  console.log('write success')
})

parallel

miss.parallel(concurrency, each)

This works like through except you can process items in parallel, while still preserving the original input order.

This is handy if you wanna take advantage of node's async I/O and process streams of items in batches. With this module you can build your very own streaming parallel job queue.

Note that miss.parallel preserves input ordering. Passing the option {ordered:false} will output the data as soon as it's processed by a transform, without waiting to respect the order (this previously required a separate module through2-concurrent).

original module

miss.parallel is provided by require('parallel-transform')

example

This example fetches the GET HTTP headers for a stream of input URLs 5 at a time in parallel.

function getResponse (item, cb) {
  var r = request(item.url)
  r.on('error', function (err) {
    cb(err)
  })
  r.on('response', function (re) {
    cb(null, {url: item.url, date: new Date(), status: re.statusCode, headers: re.headers})
    r.abort()
  })
}

miss.pipe(
  fs.createReadStream('./urls.txt'), // one url per line
  split(),
  miss.parallel(5, getResponse),
  miss.through(function (row, enc, next) {
    console.log(JSON.stringify(row))
    next()
  })
)

see also

• substack/stream-handbook
• nodejs.org/api/stream.html
• awesome-nodejs-streams

license

Licensed under the BSD 2-clause license.