through2

What is through2?

The through2 npm package is a thin wrapper around Node.js streams.Transform (a standard Node.js API) that makes it easier to create transform streams. It is designed to work with streams in object mode or non-object mode, allowing for simple function-based stream transformation. This package is particularly useful when you need to create a custom stream that modifies or transforms data as it passes through the stream pipeline.

What are through2's main functionalities?

Simple Transform

This feature allows you to create a simple transform stream that converts input data to uppercase. The code sample demonstrates how to use through2 to create a stream that reads from stdin, transforms the data, and writes to stdout.

const through2 = require('through2');
const stream = through2(function(chunk, enc, callback) {
  this.push(chunk.toString().toUpperCase());
  callback();
});
process.stdin.pipe(stream).pipe(process.stdout);

Object Mode Transform

This feature allows you to work with streams in object mode, where the stream chunks are JavaScript objects. The code sample shows how to create a transform stream that modifies properties of objects and how to use it with a readable stream of objects.

const through2 = require('through2').obj;
const stream = through2(function(obj, enc, callback) {
  obj.key = obj.key.toUpperCase();
  this.push(obj);
  callback();
});

// Usage with an array of objects
const objects = [{ key: 'value' }, { key: 'data' }];
const objectStream = require('stream').Readable.from(objects);
objectStream.pipe(stream).on('data', (transformedObj) => console.log(transformedObj));

Flush Function

This feature allows you to perform an action when the stream is ending, just before it finishes. The code sample demonstrates how to append a message to the stream output right before the stream ends.

const through2 = require('through2');
const stream = through2(function(chunk, enc, callback) {
  this.push(chunk);
  callback();
}, function(callback) {
  // This function is called before the stream is finished
  this.push('Stream is ending!');
  callback();
});

process.stdin.pipe(stream).pipe(process.stdout);

Other packages similar to through2

through

The 'through' package provides a simple way to create a readable-writable stream. It is similar to through2 but does not support object mode by default and has a less modern API.

stream-transform

The 'stream-transform' package is part of the Node.js CSV project and provides a standard transform stream with a simpler API for synchronous operations. It is specifically designed for CSV data transformation and is less generic compared to through2.

pumpify

The 'pumpify' package combines an array of streams into a single duplex stream using pump and duplexify. It is similar to through2 in that it deals with stream transformation but focuses on combining streams rather than creating individual transform streams.

README

through2

A tiny wrapper around Node.js streams.Transform (Streams2/3) to avoid explicit subclassing noise

Inspired by Dominic Tarr's through in that it's so much easier to make a stream out of a function than it is to set up the prototype chain properly: through(function (chunk) { ... }).

fs.createReadStream('ex.txt')
  .pipe(through2(function (chunk, enc, callback) {
    for (let i = 0; i < chunk.length; i++)
      if (chunk[i] == 97)
        chunk[i] = 122 // swap 'a' for 'z'

    this.push(chunk)

    callback()
   }))
  .pipe(fs.createWriteStream('out.txt'))
  .on('finish', () => doSomethingSpecial())

Or object streams:

const all = []

fs.createReadStream('data.csv')
  .pipe(csv2())
  .pipe(through2.obj(function (chunk, enc, callback) {
    const data = {
        name    : chunk[0]
      , address : chunk[3]
      , phone   : chunk[10]
    }
    this.push(data)

    callback()
  }))
  .on('data', (data) => {
    all.push(data)
  })
  .on('end', () => {
    doSomethingSpecial(all)
  })

Note that through2.obj(fn) is a convenience wrapper around through2({ objectMode: true }, fn).

Do you need this?

Since Node.js introduced Simplified Stream Construction, many uses of through2 have become redundant. Consider whether you really need to use through2 or just want to use the 'readable-stream' package, or the core 'stream' package (which is derived from 'readable-stream'):

const { Transform } = require('readable-stream')

const transformer = new Transform({
  transform(chunk, enc, callback) {
    // ...
  }
})

API

through2([ options, ] [ transformFunction ] [, flushFunction ])

Consult the stream.Transform documentation for the exact rules of the transformFunction (i.e. this._transform) and the optional flushFunction (i.e. this._flush).

options

The options argument is optional and is passed straight through to stream.Transform. So you can use objectMode:true if you are processing non-binary streams (or just use through2.obj()).

The options argument is first, unlike standard convention, because if I'm passing in an anonymous function then I'd prefer for the options argument to not get lost at the end of the call:

fs.createReadStream('/tmp/important.dat')
  .pipe(through2({ objectMode: true, allowHalfOpen: false },
    (chunk, enc, cb) => {
      cb(null, 'wut?') // note we can use the second argument on the callback
                       // to provide data as an alternative to this.push('wut?')
    }
  ))
  .pipe(fs.createWriteStream('/tmp/wut.txt'))

transformFunction

The transformFunction must have the following signature: function (chunk, encoding, callback) {}. A minimal implementation should call the callback function to indicate that the transformation is done, even if that transformation means discarding the chunk.

To queue a new chunk, call this.push(chunk)—this can be called as many times as required before the callback() if you have multiple pieces to send on.

Alternatively, you may use callback(err, chunk) as shorthand for emitting a single chunk or an error.

If you do not provide a transformFunction then you will get a simple pass-through stream.

flushFunction

The optional flushFunction is provided as the last argument (2nd or 3rd, depending on whether you've supplied options) is called just prior to the stream ending. Can be used to finish up any processing that may be in progress.

fs.createReadStream('/tmp/important.dat')
  .pipe(through2(
    (chunk, enc, cb) => cb(null, chunk), // transform is a noop
    function (cb) { // flush function
      this.push('tacking on an extra buffer to the end');
      cb();
    }
  ))
  .pipe(fs.createWriteStream('/tmp/wut.txt'));

through2.ctor([ options, ] transformFunction[, flushFunction ])

Instead of returning a stream.Transform instance, through2.ctor() returns a constructor for a custom Transform. This is useful when you want to use the same transform logic in multiple instances.

const FToC = through2.ctor({objectMode: true}, function (record, encoding, callback) {
  if (record.temp != null && record.unit == "F") {
    record.temp = ( ( record.temp - 32 ) * 5 ) / 9
    record.unit = "C"
  }
  this.push(record)
  callback()
})

// Create instances of FToC like so:
const converter = new FToC()
// Or:
const converter = FToC()
// Or specify/override options when you instantiate, if you prefer:
const converter = FToC({objectMode: true})

License

through2 is Copyright © Rod Vagg and additional contributors and licensed under the MIT license. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE file for more details.