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L'objecte Promise representa l'eventual finalització (o fracàs) d'una operació asincrònica i el seu valor resultant.

Aquesta entrada és per al constructor Promise. Per obtenir informació sobre les promeses, llegeix primer Emprant promises. El constructor s'utilitza principalment per embolcallar funcions que no admeten promises.


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


Una funció que es passa amb els arguments resolve i reject. La funció executor s'executa immediatament mitjançant la implementació de la Promesa, passant les funcions resolve i reject (l'executor es crida fins i tot abans que el constructor Promise retorni l'objecte creat). Les funcions de resolve i reject, quan es criden, resolen o rebutgen la promesa, respectivament. Normalment, l'executor inicia alguns treballs asincrònics, i després, un cop finalitzi, o bé crida a la funció resolve per resoldre la promesa, o bé la rebutja si s'ha produït un error.
Si es produeix un error en la funció de l'executor, la promesa és rebutjada. El valor retornat de l'executor és ignorat.


Una Promise és un proxy per a un valor no necessariàment conegut quan la promesa és creada. Això us permet associar gestors amb l'eventual valor exitós o la raó de fallida d'una acció asincrònica. Això permet als mètodes asincrònics retornar valors de manera semblant als mètodes sincrònics: en lloc de retornar immediatament el valor final, el mètode asincrònic retorna una promesa que proveirà el valor en algun punt en el futur. 

Una Promise pot tenir un dels següents estats:

  • pending: estat inicial, ni complerta ni rebutjada.
  • fulfilled: indicant que l'operació s'ha completat amb èxit.
  • rejected: indicant que l'operació ha fallat.

Una promesa pendent pot serfulfilled amb un valor, o bé rejected amb un motiu (error). Quan alguna d'aquestes opcions succeeix, els gestors associats concatenats pel mètode then de la promesa són executats. (Si la promesa ja ha sigut complerta o rebutjada quan un gestor corresponent és adjuntat, el gestor serà cridat, evitant així un estat de competició entre una operació asíncrona essent completada i llurs gestors essent adjuntats.

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

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 fulfilled. States and fates contains more details about promise terminology.


Length property whose value is always 1 (number of constructor arguments).
Represents the prototype for the Promise constructor.


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


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


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.
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).
Appends a handler to the promise, and returns a new promise which is resolved when the original promise is resolved. The handler is called when the promise is settled, whether fulfilled or rejected.

Creating a Promise

A Promise object is created using the new keyword and its constructor. This constructor takes as its argument a function, called the "executor function". This function should take two functions as parameters. The first of these functions (resolve) is called when the asynchronous task completes successfully and returns the results of the task as a value. The second (reject) is called when the task fails, and returns the reason for failure, which is typically an error object.

const myFirstPromise = new Promise((resolve, reject) => {
  // do something asynchronous which eventually calls either:
  //   resolve(someValue); // fulfilled
  // or
  //   reject("failure reason"); // rejected

To provide a function with promise functionality, simply have it return a promise:

function myAsyncFunction(url) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest();"GET", url);
    xhr.onload = () => resolve(xhr.responseText);
    xhr.onerror = () => reject(xhr.statusText);


Basic Example

let myFirstPromise = new Promise((resolve, reject) => {
  // We call resolve(...) when what we were doing asynchronously was successful, and reject(...) when it failed.
  // In this example, we use setTimeout(...) to simulate async code. 
  // In reality, you will probably be using something like XHR or an HTML5 API.
    resolve("Success!"); // Yay! Everything went well!
  }, 250);

myFirstPromise.then((successMessage) => {
  // successMessage is whatever we passed in the resolve(...) function above.
  // It doesn't have to be a string, but if it is only a succeed message, it probably will be.
  console.log("Yay! " + successMessage);

Advanced Example

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 be fulfilled, 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 show how the synchronous part of the method is decoupled from the asynchronous completion of the promise.

'use strict';
var promiseCount = 0;

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

    let 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)
    let p1 = new Promise(
        // The resolver function is called with the ability to resolve or
        // reject the promise
       (resolve, reject) => {
            log.insertAdjacentHTML('beforeend', thisPromiseCount +
                ') Promise started (<small>Async code started</small>)<br/>');
            // This is only an example to create asynchronism
                function() {
                    // We fulfill the promise !
                }, Math.random() * 2000 + 1000);

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

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

This example is started by clicking the button. You need a browser that supports Promise. By clicking the button several times in a short amount of time, you'll even see the different promises being fulfilled one after another.

Loading an image with XHR

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


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

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
Promise() constructorChrome Full support 32Edge Full support YesFirefox Full support 29
Full support 29
Notes Constructor requires a new operator since version 37.
IE No support NoOpera Full support 19Safari Full support 8
Full support 8
Notes Constructor requires a new operator since version 10.
WebView Android Full support 4.4.3Chrome Android Full support 32Edge Mobile Full support YesFirefox Android Full support 29
Full support 29
Notes Constructor requires a new operator since version 37.
Opera Android Full support YesSafari iOS Full support 8
Full support 8
Notes Constructor requires a new operator since version 10.
Samsung Internet Android Full support Yesnodejs Full support 0.12
Full support 0.12
Notes Constructor requires a new operator since version 4.


Full support  
Full support
No support  
No support
See implementation notes.
See implementation notes.

See also

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