when

What is when?

The 'when' npm package is a robust library for working with asynchronous programming in JavaScript, particularly using promises. It provides utilities for creating, managing, and composing promises, making it easier to handle asynchronous operations and their potential complexities.

What are when's main functionalities?

Creating Promises

This feature allows the creation of new promises. The code sample demonstrates how to create a simple promise that resolves with 'Hello, World!' after 1 second.

const when = require('when');
const promise = when.promise(function(resolve, reject) {
  setTimeout(() => resolve('Hello, World!'), 1000);
});
promise.then(response => console.log(response));

Chaining Promises

This feature demonstrates chaining multiple promises. It shows how to perform a series of tasks sequentially, where each task starts only after the previous one has completed.

const when = require('when');
const cleanRoom = () => when.promise(resolve => resolve('Room cleaned'));
const removeTrash = () => when.promise(resolve => resolve('Trash removed'));
const winIcecream = () => when.promise(resolve => resolve('Won ice cream'));
cleanRoom()
  .then(result => {
    console.log(result);
    return removeTrash();
  })
  .then(result => {
    console.log(result);
    return winIcecream();
  })
  .then(result => console.log(result));

Handling Errors

This feature involves error handling in promises. The code sample shows how to catch and handle errors that occur during the execution of promises.

const when = require('when');
const failTask = () => when.promise((resolve, reject) => reject('Failed task'));
failTask()
  .then(result => console.log('Success:', result))
  .catch(error => console.log('Error:', error));

Other packages similar to when

bluebird

Bluebird is a full-featured promise library with a focus on innovative features and performance. It is similar to 'when' but often cited for its superior performance and additional features like cancellation, progress tracking, and more detailed stack traces.

q

Q is one of the earliest promise libraries that influenced many others. It offers a similar API to 'when' but is generally considered to be less performant in modern applications. It provides a straightforward approach to handling asynchronous operations with promises.

promise

The 'promise' package provides a minimalist implementation similar to the ES6 Promise specification. It is more lightweight compared to 'when' but lacks some of the more advanced features and utilities provided by 'when'.

README

when.js

When.js is cujoJS's lightweight Promises/A+ and when() implementation that powers the async core of wire.js, cujoJS's IOC Container. It features:

• A rock solid, battle-tested Promise implementation
• Resolving, settling, mapping, and reducing arrays of promises
• Executing tasks in parallel and sequence
• Transforming Node-style and other callback-based APIs into promise-based APIs

It passes the Promises/A+ Test Suite, is very fast and compact, and has no external dependencies.

What's New?

2.3.0

• New promise.tap for adding side effects to a promise chain.
• New MessageChannel scheduler reduces "time-to-first" handler, in environments that support it.
• Performance optimizations for promise resolution.
• Internal architecture improvements to pave the way for when.js 3.0.0.

2.2.1

• Fix for when.defer().reject() bypassing the unhandled rejection monitor. (#166)
• Fix for when/function, when/callbacks, and when/node/function not preserving thisArg. (#162)
• Doc clarifications for promise.yield. (#164)

2.2.0

• New experimental promise monitoring and debugging via when/monitor/console.
• New when.promise(resolver) promise creation API. A lighter alternative to the heavier when.defer()
• New bindCallback and liftCallback in when/node/function for more integration options with node-style callbacks.

2.1.1

• Quote internal usages of promise.yield to workaround .NET minifier tools that don't yet understand ES5 identifier-as-property rules. See #157

2.1.0

• New when.settle that settles an array of promises
• New when/guard generalized concurrency guarding and limiting
• New promise.inspect for synchronously getting a snapshot of a promise's state at a particular instant.
• Significant performance improvements when resolving promises with non-primitives (e.g. with Arrays, Objects, etc.)
• Experimental vert.x support
• DEPRECATED: onFulfilled, onRejected, onProgress handler arguments to when.all, when.any, when.some. Use the returned promise's then() (or otherwise(), ensure(), etc) to register handlers instead.
  • For example, do this: when.all(array).then(onFulfilled, onRejected) instead of this: when.all(array, onFulfilled, onRejected). The functionality is equivalent.

2.0.1

• Account for the fact that Mocha creates a global named process. Thanks Narsul

2.0.0

• Fully asynchronous resolutions.
• Promises/A+ compliance.
• New when/keys module with all() and map() for object keys/values.
• New promise.ensure as a better, and safer, replacement for promise.always. See discussion as to why promise.always is mistake-prone.
  • DEPRECATED: promise.always
• lift() is now the preferred name for what was bind() in when/function, when/node/function, and when/callbacks.
  • DEPRECATED: bind() in when/function, when/node/function, and when/callbacks. Use lift() instead.

Full Changelog

Docs & Examples

API docs

More info on the wiki

Examples

Quick Start

AMD

1. Get it

   • bower install when or yeoman install when, or
   • git clone https://github.com/cujojs/when or git submodule add https://github.com/cujojs/when

2. Configure your loader with a package:

   packages: [
   	{ name: 'when', location: 'path/to/when/', main: 'when' },
   	// ... other packages ...
   ]
   

3. define(['when', ...], function(when, ...) { ... }); or require(['when', ...], function(when, ...) { ... });

Node

1. npm install when
2. var when = require('when');

RingoJS

1. ringo-admin install cujojs/when
2. var when = require('when');

Legacy environments

1. git clone https://github.com/cujojs/when or git submodule add https://github.com/cujojs/when

2. Add a transient define shim, and a <script> element for when.js

   <script>
   	window.define = function(factory) {
   		try{ delete window.define; } catch(e){ window.define = void 0; } // IE
   		window.when = factory();
   	};
   	window.define.amd = {};
   </script>
   <script src="path/to/when/when.js"></script>
   

3. when will be available as window.when

Running the Unit Tests

Node

Note that when.js includes the Promises/A+ Test Suite. Running unit tests in Node will run both when.js's own test suite, and the Promises/A+ Test Suite.

1. npm install
2. npm test

Browsers

1. npm install
2. npm start - starts buster server & prints a url
3. Point browsers at /capture, e.g. localhost:1111/capture
4. npm run-script test-browser

References

Much of this code was inspired by the async innards of wire.js, and has been influenced by the great work in Q, Dojo's Deferred, and uber.js.

Changelog

2.3.0

• New promise.tap for adding side effects to a promise chain.
• New MessageChannel scheduler reduces "time-to-first" handler, in environments that support it.
• Performance optimizations for promise resolution.
• Internal architecture improvements to pave the way for when.js 3.0.0.