Promise

The Promise object is used for deferred and asynchronous computations. A Promise is in one of these states:

• pending: initial state, not fulfilled or rejected.
• fulfilled: successful operation
• rejected: failed operation.
• settled: the Promise is either fulfilled or rejected, but not pending.

Syntax

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

Parameters

executor

Function object with two arguments resolve and reject. The first argument fulfills the promise, the second argument rejects it. We can call these functions, once our operation is completed.

Description

The Promise interface 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 or failure. 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 pending promise can become either fulfilled with a value, or rejected with a reason. 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.

Properties

Promise.length

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

Promise.prototype

Represents the prototype for the Promise constructor.

Methods

Promise.all(iterable)

Returns a promise that resolves when all of the promises in the iterable argument have resolved.

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.

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.

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 resolve, using window.setTimeout, to the string 'result' after 1s to 3s (random).

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.

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 the string 'result' (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 only is 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 fulfilled
  p1.then(
    // Just log the message and a value
    function(val) {
      log.insertAdjacentHTML('beforeend', val +
          ') Promise fulfilled (<small>Async code terminated</small>)<br/>');
    });

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

if ("Promise" in window) {
  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 Promise 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 promise being fulfilled one after the other.

Example using new XMLHttpRequest()

Creating a Promise

This example shows the implementation of a method of success callback and error in XMLHttpRequest Interestingly enough.

'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) {
            // Performs the function "resolve" when this.status is equal to 200
            resolve(this.response);
          } else {
            // Performs the function "reject" when this.status is different than 200
            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);

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

[1] Gecko 24 has an experimental implementation of Promise, under the initial name of Future. It was renamed to its final name in Gecko 25, but disabled by default behind the flag dom.promise.enabled. Bug 918806 enabled Promises by default in Gecko 29.

See also

• Promises/A+ specification
• 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