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The Promise object is used for asynchronous computations. A Promise represents a single asynchronous operation that hasn't completed yet, but is expected in the future.

Syntax

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

Parameters

executor
A function that will be passing other functions via the arguments resolve and reject. The executor function is executed immediately by the Promise implementation which provides the resolve and reject functions (the executor is called before the Promise constructor even returns the created object). The resolve and reject functions are bound to the promise and calling them fulfills or rejects the promise, respectively. The executor is expected to initiate some asynchronous work and then, once that completes, call either the resolve or reject function to resolve the promise's final value or else reject it if an error occurred.

Description

A Promise represents 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 of having a 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 become either 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—an operation called composition.

Not to be confused with: Several other languages have mechanisms for lazy evaluation and deferring a computation, which they also call “promises” — e.g. Scheme. Promises in JavaScript represent processes which are already happening, which can be chained with callback functions. If you are looking to lazily evaluate an expression, consider the arrow function with no arguments: f = () => expression to create the lazily-evaluated expression, and f() to evaluate.

Note: A promise is said to be settled if it is either fulfilled or rejected, but not pending. You will also hear the term resolved used with promises — this means that the promise is settled, or it is locked into a promise chain. Domenic Denicola's States and fates contains more details about promise terminology.

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 resolves when all of the promises in the iterable argument have resolved or rejects as soon as one of the promises in the iterable argument rejects. If the returned promise resolves, it is resolved with an array of the values from the resolved promises in same order as defined in the iterable. If the returned promise rejects, it is rejected with the reason from the promise in the iterable that rejected. This method can be useful for aggregating results of multiple promises together.
Promise.race(iterable)
Returns a promise that resolves or rejects as soon as one of the promises in the iterable resolves 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 want to 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

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 resolve, 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/>');
}

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 success 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

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Promise' in that specification.
Standard Initial definition in an ECMA standard.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Promise' in that specification.
Draft  

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari Servo
Promise32.0(Yes)29.0No support197.1No support
Constructor requires new32.0(Yes)37.0No support1910No support
Promise.all32.0(Yes)29.0No support197.1No support
Promise.prototype32.0(Yes)29.0No support197.1No support
Promise.prototype.catch32.0(Yes)29.0No support197.1No support
Promise.prototype.then32.0(Yes)29.0No support197.1No support
Promise.race32.0(Yes)29.0No support197.1No support
Promise.reject32.0(Yes)29.0No support197.1No support
Promise.resolve32.0(Yes)29.0No support197.1No support
Feature Android Chrome for Android Edge Mobile Firefox for Android IE Mobile Opera Mobile Safari Mobile
Promise4.4.432.0(Yes)29No support(Yes)8.0
Constructor requires new4.4.432.0(Yes)37.0No support(Yes)10
Promise.all4.4.432.0(Yes)29No support(Yes)8.0
Promise.prototype4.4.432.0(Yes)29No support(Yes)8.0
Promise.prototype.catch4.4.432.0(Yes)29No support(Yes)8.0
Promise.prototype.then4.4.432.0(Yes)29No support(Yes)8.0
Promise.race4.4.432.0(Yes)29No support(Yes)8.0
Promise.reject4.4.432.0(Yes)29No support(Yes)8.0
Promise.resolve4.4.432.0(Yes)29No support(Yes)8.0

See also