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
• API docs
• Examples
• More info on the wiki

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');

Ender

1. ender add cujojs/when
2. var when = require('when');

Legacy environments (via browserify)

1. git clone https://github.com/cujojs/when
2. npm install
3. npm run browserify to generate build/when.js
   1. Or npm run browserify-debug to build with when/monitor/console enabled
4. <script src="path/to/when/build/when.js"></script>
   1. when will be available as window.when
   2. Other modules will be available as sub-objects/functions, e.g. window.when.fn.lift, window.when.sequence. See the full sub-namespace list in the browserify build file

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

2.7.0

• Added promise.catch and promise.finally as synonyms for promise.otherwise and promise.ensure. (#212)
• New browserify build for those using globals. (#209)
• Added ender support to package.json. (#223)
• Fix compatibility with PhantomJS's CommonJS module support. (#226)
• Fix Sauce Labs tests for pull requests. (#216)
• Added bower.json ignore to trim files installed via bower. (#193)