Promise

The Promise object is used for asynchronous computations. A Promise represents a value which may be available now, or in the future, or never.

Syntax

new Promise( /* executor */ function(resolve, reject) { ... } );

Parameters

executor

A function that is passed with the arguments resolve and reject. The executor function is executed immediately by the Promise implementation, passing resolve and reject functions (the executor is called before the Promise constructor even returns the created object). The resolve and reject functions, when called, resolve or reject the promise respectively. The executor normally initiates some asynchronous work and then, once that completes, calls either the resolve or reject function to resolve the promise or else reject it if an error occurred.

Description

A Promise is a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers to an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of the final value, the asynchronous method returns a promise for the value at some point in the future.

A Promise is in one of these states:

• pending: initial state, not fulfilled or rejected.
• fulfilled: meaning that the operation completed successfully.
• rejected: meaning that the operation failed.

A pending promise can either be fulfilled with a value, or rejected with a reason (error). When either of these happens, the associated handlers queued up by a promise's then method are called. (If the promise has already been fulfilled or rejected when a corresponding handler is attached, the handler will be called, so there is no race condition between an asynchronous operation completing and its handlers being attached.)

As the Promise.prototype.then() and Promise.prototype.catch() methods return promises, they can be chained.

Properties

Promise.length

Length property whose value is always 1 (number of constructor arguments).

Promise.prototype

Represents the prototype for the Promise constructor.

Methods

Promise.all(iterable)

Returns a promise that either fulfills when all of the promises in the iterable argument have fulfilled or rejects as soon as one of the promises in the iterable argument rejects. If the returned promise fulfills, it is fulfilled with an array of the values from the fulfilled promises in same order as defined in the iterable. If the returned promise rejects, it is rejected with the reason from the first promise in the iterable that rejected. This method can be useful for aggregating results of multiple promises.

Promise.race(iterable)

Returns a promise that fulfills or rejects as soon as one of the promises in the iterable fulfills or rejects, with the value or reason from that promise.

Promise.reject(reason)

Returns a Promise object that is rejected with the given reason.

Promise.resolve(value)

Returns a Promise object that is resolved with the given value. If the value is a thenable (i.e. has a then method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value. Generally, if you don't know if a value is a promise or not, Promise.resolve(value) it instead and work with the return value as a promise.

Promise prototype

Properties

Promise.prototype.constructor

Returns the function that created an instance's prototype. This is the Promise function by default.

Methods

Promise.prototype.catch(onRejected)

Appends a rejection handler callback to the promise, and returns a new promise resolving to the return value of the callback if it is called, or to its original fulfillment value if the promise is instead fulfilled.

Promise.prototype.then(onFulfilled, onRejected)

Appends fulfillment and rejection handlers to the promise, and returns a new promise resolving to the return value of the called handler, or to its original settled value if the promise was not handled (i.e. if the relevant handler onFulfilled or onRejected is not a function).

Examples

Creating a Promise

<button id="btn">Make a promise!</button>
<div id="log"></div>

This small example shows the mechanism of a Promise. The testPromise() method is called each time the <button> is clicked. It creates a promise that will fulfill, using window.setTimeout(), to the promise count (number starting from 1) every 1-3 seconds, at random. The Promise() constructor is used to create the promise.

The fulfillment of the promise is simply logged, via a fulfill callback set using p1.then(). A few logs shows how the synchronous part of the method is decoupled of the asynchronous completion of the promise.

'use strict';
var promiseCount = 0;

function testPromise() {
    var thisPromiseCount = ++promiseCount;

    var log = document.getElementById('log');
    log.insertAdjacentHTML('beforeend', thisPromiseCount +
        ') Started (<small>Sync code started</small>)<br/>');

    // We make a new promise: we promise a numeric count of this promise, starting from 1 (after waiting 3s)
    var p1 = new Promise(
        // The resolver function is called with the ability to resolve or
        // reject the promise
        function(resolve, reject) {
            log.insertAdjacentHTML('beforeend', thisPromiseCount +
                ') Promise started (<small>Async code started</small>)<br/>');
            // This is only an example to create asynchronism
            window.setTimeout(
                function() {
                    // We fulfill the promise !
                    resolve(thisPromiseCount);
                }, Math.random() * 2000 + 1000);
        }
    );

    // We define what to do when the promise is resolved/fulfilled with the then() call,
    // and the catch() method defines what to do if the promise is rejected.
    p1.then(
        // Log the fulfillment value
        function(val) {
            log.insertAdjacentHTML('beforeend', val +
                ') Promise fulfilled (<small>Async code terminated</small>)<br/>');
        })
    .catch(
        // Log the rejection reason
        function(reason) {
            console.log('Handle rejected promise ('+reason+') here.');
        });

    log.insertAdjacentHTML('beforeend', thisPromiseCount +
        ') Promise made (<small>Sync code terminated</small>)<br/>');
}

if ("Promise" in window) {
  var btn = document.getElementById("btn");
  btn.addEventListener("click",testPromise);
} else {
  log = document.getElementById('log');
  log.innerHTML = "Live example not available as your browser doesn't support the <code>Promise<code> interface.";
}

This example is executed when clicking the button. You need a browser supporting Promise. By clicking several times the button in a short amount of time, you'll even see the different promises being fulfilled one after the other.

Example using XMLHttpRequest

Creating a Promise

This example shows the implementation of a method which uses a Promise to report the result of (or failure of) an XMLHttpRequest.

'use strict';

// A-> $http function is implemented in order to follow the standard Adapter pattern
function $http(url){
 
  // A small example of object
  var core = {

    // Method that performs the ajax request
    ajax: function (method, url, args) {

      // Creating a promise
      var promise = new Promise( function (resolve, reject) {

        // Instantiates the XMLHttpRequest
        var client = new XMLHttpRequest();
        var uri = url;

        if (args && (method === 'POST' || method === 'PUT')) {
          uri += '?';
          var argcount = 0;
          for (var key in args) {
            if (args.hasOwnProperty(key)) {
              if (argcount++) {
                uri += '&';
              }
              uri += encodeURIComponent(key) + '=' + encodeURIComponent(args[key]);
            }
          }
        }

        client.open(method, uri);
        client.send();

        client.onload = function () {
          if (this.status >= 200 && this.status < 300) {
            // Performs the function "resolve" when this.status is equal to 2xx
            resolve(this.response);
          } else {
            // Performs the function "reject" when this.status is different than 2xx
            reject(this.statusText);
          }
        };
        client.onerror = function () {
          reject(this.statusText);
        };
      });

      // Return the promise
      return promise;
    }
  };

  // Adapter pattern
  return {
    'get': function(args) {
      return core.ajax('GET', url, args);
    },
    'post': function(args) {
      return core.ajax('POST', url, args);
    },
    'put': function(args) {
      return core.ajax('PUT', url, args);
    },
    'delete': function(args) {
      return core.ajax('DELETE', url, args);
    }
  };
};
// End A

// B-> Here you define its functions and its payload
var mdnAPI = 'https://developer.mozilla.org/en-US/search.json';
var payload = {
  'topic' : 'js',
  'q'     : 'Promise'
};

var callback = {
  success: function(data) {
    console.log(1, 'success', JSON.parse(data));
  },
  error: function(data) {
    console.log(2, 'error', JSON.parse(data));
  }
};
// End B

// Executes the method call 
$http(mdnAPI) 
  .get(payload) 
  .then(callback.success) 
  .catch(callback.error);

// Executes the method call but an alternative way (1) to handle Promise Reject case 
$http(mdnAPI) 
  .get(payload) 
  .then(callback.success, callback.error);

// Executes the method call but an alternative way (2) to handle Promise Reject case 
$http(mdnAPI) 
  .get(payload) 
  .then(callback.success)
  .then(undefined, callback.error);

Loading an image with XHR

Another simple example using Promise and XMLHttpRequest to load an image is available at the MDN GitHub promise-test repository. You can also see it in action. Each step is commented and allows you to follow the Promise and XHR architecture closely.

Specifications

Browser compatibility

To contribute to this compatibility data, please write a pull request against this file: https://github.com/mdn/browser-compat-data/blob/master/javascript/promise.json.

1. Constructor requires a new operator since version 37.

2. Constructor requires a new operator since version 10.

See also

• Promises/A+ specification
• Venkatraman.R - JS Promise (Part 1, Basics)
• Jake Archibald: JavaScript Promises: There and Back Again
• Domenic Denicola: Callbacks, Promises, and Coroutines – Asynchronous Programming Patterns in JavaScript
• Matt Greer: JavaScript Promises ... In Wicked Detail
• Forbes Lindesay: promisejs.org
• Nolan Lawson: We have a problem with promises — Common mistakes with promises
• Promise polyfill
• Udacity: JavaScript Promises