ES6 Promise Polyfill

Promises allow you to interact with a value that may or may not be available yet.

Getting Started

This polyfill can be loaded as:

• A script that acts as a polyfill for native promises and adds a global Promise constructor if the native version is not available
• A Node.js module available in npm
• An AMD module
• As part of the YUI library. See its User Guide

Node.js

Installation

To use this module in Node.js, add the ypromise module to your dependencies in the package.json file of your project:

{
	"dependencies": {
		"ypromise": "[email protected]:yahoo/ypromise"
	}
}

Install it using npm:

$ npm install

Usage

The ypromise module exports the Promise constructor:

var Promise = require('ypromise');

function asyncFunction() {
	return new Promise(function (resolve, reject) {
		resolve('Hello world');
	});
}

Promise API reference

Constructor

new Promise(function (resolve, reject) {});

resolve(value)

If value is a promise or a thenable, the new promise will adopt its value once it settles.

reject(reason)

Your promise is rejected with reason. For consistency and debugging it is encouraged that reason is an instance of Error.

Instance methods

promise.then(onFulfilled, onRejected)

onFulfilled is called when/if "promise" resolves. onRejected is called when/if "promise" rejects. Both are optional, if either/both are omitted the next onFulfilled/onRejected in the chain is called. Both callbacks have a single parameter, the fulfillment value or rejection reason. then returns a new promise equivalent to the value you return from onFulfilled/onRejected after being passed through Promise.resolve. If an error is thrown in the callback, the returned promise rejects with that error.

promise.catch(onRejected)

Sugar for promise.then(undefined, onRejected).

Static methods

Promise.resolve(value)

Always returns a promise. If value is a promise and its constructor is Promise resolve will return it without modifying it. Otherwise, resolve will return a new promise that resolves to value.

Promise.reject(reason)

Returns a promise rejected with reason. Sugar for new Promise(function (resolve, reject) { reject(value); }).

Promise.all(list)

Returns a promise that fulfills when every item in the array fulfills, and rejects if (and when) any item rejects. Each array item is passed to Promise.resolve, so the array can be a mixture of promise-like objects and other objects. The fulfillment value is an array (in order) of fulfillment values. The rejection value is the first rejection value.

Promise.race(list)

Returns a Promise that fulfills when any item fulfills, and rejects if any item rejects. Essentially, the first promise to be settled wins the race.

License

This software is free to use under the Yahoo! Inc. BSD license. See the LICENSE file for license text and copyright information.

Contribute

See the CONTRIBUTING file for info.