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async-transforms v1.0.3

Asynchronous stream transforms


Asynchronous stream transforms for Node. Allows async handlers and parallel execution, useful for build systems like Gulp and friends.


Install async-transforms with your favourite package manager.


Use, .filter or .gate to generate a stream.Transform instance that calls the passed handler.

// for example

import * as transforms from 'async-transforms';
import 'stream';

  stream.Readable.from([object1, object2]),  // used for demo (object) => {
    await object.expensiveOperation;
    await object.someOtherThing;
  (err) => {
    // callback

These transforms operate in parallel and don't guarantee the order of their output (whatever finishes first). You can set options to configure behavior:

const s =, {
  order: true,  // force the same output order
  tasks: 5,     // limits number of parallel tasks

It's also possible to set objectMode: false (it's true by default) but this is unlikely to be useful to you.


This example uses async-transforms to parallelize rendering with Less. This is important if e.g., Less is loading further files from the filesystem.

const transforms = require('async-transforms');
const {src, dest} = require('gulp');
const less = require('less');

exports.default = () => {
  return src('*.less')
    .pipe( (file) => {
      const result = await less.render(file.contents.toString('utf8'));
      file.contents = Buffer.from(result.css);
      file.extname = '.css';

While Gulp plugins for Less already exist, this makes it easier to write general-purpose, modern plugins with async and await syntax.

Worker Pool

This includes a submodule which provides a worker pool. It's useful when combined with the above transforms handler. For example:

import {pool} from 'async-transforms/worker';

const compilationPool = pool(path.resolve('./compile.js'));

// use directly
compilationPool(123, {tasks: 2})
    .then((result) =>'result from passing value to worker', result));

// or as part of a transform
stream.Readable.from([object1, object2])
    .pipe( => {
      // do something with the result

The pool invokes the default export (or module.exports for CJS) of the target file. By default, it utilizes 75% of your local CPUs, but set tasks to control this—use a fraction from 0-1 to set a ratio, and higher for absolute.

Use this for CPU-bound tasks like JS minification.

This doesn't really belong in this module.


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