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

APromise is in one of these states:

• pending: initial state, neither fulfilled nor rejected.
• fulfilled: meaning that the operation was completed successfully.
• rejected: meaning that the operation failed.

Theeventual state of a pending promise can either befulfilled with a value orrejected with a reason (error).When either of these options occur, the associated handlers queued up by a promise'sthen 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.

A promise is said to besettled if it is either fulfilled or rejected, but not pending.

You will also hear the termresolved used with promises — this means that the promise is settled or "locked-in" to match the eventual state of another promise, and further resolving or rejecting it has no effect. TheStates and fates document from the original Promise proposal contains more details about promise terminology. Colloquially, "resolved" promises are often equivalent to "fulfilled" promises, but as illustrated in "States and fates", resolved promises can be pending or rejected as well. For example:

new Promise((resolveOuter) => {  resolveOuter(    new Promise((resolveInner) => {      setTimeout(resolveInner, 1000);    }),  );});

This promise is alreadyresolved at the time when it's created (because theresolveOuter is called synchronously), but it is resolved with another promise, and therefore won't befulfilled until 1 second later, when the inner promise fulfills. In practice, the "resolution" is often done behind the scenes and not observable, and only its fulfillment or rejection are.

Promise itself has no first-class protocol for cancellation, but you may be able to directly cancel the underlying asynchronous operation, typically usingAbortController.

The promise methodsthen(),catch(), andfinally() are used to associate further action with a promise that becomes settled. Thethen() method takes up to two arguments; the first argument is a callback function for the fulfilled case of the promise, and the second argument is a callback function for the rejected case. Thecatch() andfinally() methods callthen() internally and make error handling less verbose. For example, acatch() is really just athen() without passing the fulfillment handler. As these methods return promises, they can be chained. For example:

const myPromise = new Promise((resolve, reject) => {  setTimeout(() => {    resolve("foo");  }, 300);});myPromise  .then(handleFulfilledA, handleRejectedA)  .then(handleFulfilledB, handleRejectedB)  .then(handleFulfilledC, handleRejectedC);

We will use the following terminology:initial promise is the promise on whichthen is called;new promise is the promise returned bythen. The two callbacks passed tothen are calledfulfillment handler andrejection handler, respectively.

The settled state of the initial promise determines which handler to execute.

• If the initial promise is fulfilled, the fulfillment handler is called with the fulfillment value.
• If the initial promise is rejected, the rejection handler is called with the rejection reason.

The completion of the handler determines the settled state of the new promise.

• If the handler returns athenable value, the new promise settles in the same state as the returned value.
• If the handler returns a non-thenable value, the new promise is fulfilled with the returned value.
• If the handler throws an error, the new promise is rejected with the thrown error.
• If the initial promise has no corresponding handler attached, the new promise will settle to the same state as the initial promise — that is, without a rejection handler, a rejected promise stays rejected with the same reason.

For example, in the code above, ifmyPromise rejects,handleRejectedA will be called, and ifhandleRejectedA completes normally (without throwing or returning a rejected promise), the promise returned by the firstthen will be fulfilled instead of staying rejected. Therefore, if an error must be handled immediately, but we want to maintain the error state down the chain, we must throw an error of some type in the rejection handler. On the other hand, in the absence of an immediate need, we can leave out error handling until the finalcatch() handler.

myPromise  .then(handleFulfilledA)  .then(handleFulfilledB)  .then(handleFulfilledC)  .catch(handleRejectedAny);

Usingarrow functions for the callback functions, implementation of the promise chain might look something like this:

myPromise  .then((value) => `${value} and bar`)  .then((value) => `${value} and bar again`)  .then((value) => `${value} and again`)  .then((value) => `${value} and again`)  .then((value) => {    console.log(value);  })  .catch((err) => {    console.error(err);  });

JavaScript maintains ajob queue. Each time, JavaScript picks a job from the queue and executes it to completion. The jobs are defined by the executor of thePromise() constructor, the handlers passed tothen, or any platform API that returns a promise. The promises in a chain represent the dependency relationship between these jobs. When a promise settles, the respective handlers associated with it are added to the back of the job queue.

A promise can participate in more than one chain. For the following code, the fulfillment ofpromiseA will cause bothhandleFulfilled1 andhandleFulfilled2 to be added to the job queue. BecausehandleFulfilled1 is registered first, it will be invoked first.

const promiseA = new Promise(myExecutorFunc);const promiseB = promiseA.then(handleFulfilled1, handleRejected1);const promiseC = promiseA.then(handleFulfilled2, handleRejected2);

An action can be assigned to an already settled promise. In this case, the action is added immediately to the back of the job queue and will be performed when all existing jobs are completed. Therefore, an action for an already "settled" promise will occur only after the current synchronous code completes and at least one loop-tick has passed. This guarantees that promise actions are asynchronous.

const promiseA = new Promise((resolve, reject) => {  resolve(777);});// At this point, "promiseA" is already settled.promiseA.then((val) => console.log("asynchronous logging has val:", val));console.log("immediate logging");// produces output in this order:// immediate logging// asynchronous logging has val: 777

The JavaScript ecosystem had made multiple Promise implementations long before it became part of the language. Despite being represented differently internally, at the minimum, all Promise-like objects implement theThenable interface. A thenable implements the.then() method, which is called with two callbacks: one for when the promise is fulfilled, one for when it's rejected. Promises are thenables as well.

To interoperate with the existing Promise implementations, the language allows using thenables in place of promises. For example,Promise.resolve will not only resolve promises, but also trace thenables.

const aThenable = {  then(onFulfilled, onRejected) {    onFulfilled({      // The thenable is fulfilled with another thenable      then(onFulfilled, onRejected) {        onFulfilled(42);      },    });  },};Promise.resolve(aThenable); // A promise fulfilled with 42

ThePromise class offers four static methods to facilitate async taskconcurrency:

Promise.all()

Fulfills whenall of the promises fulfill; rejects whenany of the promises rejects.

Promise.allSettled()

Fulfills whenall promises settle.

Promise.any()

Fulfills whenany of the promises fulfills; rejects whenall of the promises reject.

Promise.race()

Settles whenany of the promises settles. In other words, fulfills when any of the promises fulfills; rejects when any of the promises rejects.

All these methods take aniterable of promises (thenables, to be exact) and return a new promise. They all support subclassing, which means they can be called on subclasses ofPromise, and the result will be a promise of the subclass type. To do so, the subclass's constructor must implement the same signature as thePromise() constructor — accepting a singleexecutor function that can be called with theresolve andreject callbacks as parameters. The subclass must also have aresolve static method that can be called likePromise.resolve() to resolve values to promises.

Note that JavaScript issingle-threaded by nature, so at a given instant, only one task will be executing, although control can shift between different promises, making execution of the promises appear concurrent.Parallel execution in JavaScript can only be achieved throughworker threads.

Promise()

Creates a newPromise object. The constructor is primarily used to wrap functions that do not already support promises.

Promise[Symbol.species]

Returns the constructor used to construct return values from promise methods.

Promise.all()

Takes an iterable of promises as input and returns a singlePromise. This returned promise fulfills when all of the input's promises fulfill (including when an empty iterable is passed), with an array of the fulfillment values. It rejects when any of the input's promises reject, with this first rejection reason.

Promise.allSettled()

Takes an iterable of promises as input and returns a singlePromise. This returned promise fulfills when all of the input's promises settle (including when an empty iterable is passed), with an array of objects that describe the outcome of each promise.

Promise.any()

Takes an iterable of promises as input and returns a singlePromise. This returned promise fulfills when any of the input's promises fulfill, with this first fulfillment value. It rejects when all of the input's promises reject (including when an empty iterable is passed), with anAggregateError containing an array of rejection reasons.

Promise.race()

Takes an iterable of promises as input and returns a singlePromise. This returned promise settles with the eventual state of the first promise that settles.

Promise.reject()

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

Promise.resolve()

Returns aPromise object that is resolved with the given value. If the value is a thenable (i.e., has athen method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise, the returned promise will be fulfilled with the value.

Promise.try()

Takes a callback of any kind (returns or throws, synchronously or asynchronously) and wraps its result in aPromise.

Promise.withResolvers()

Returns an object containing a newPromise object and two functions to resolve or reject it, corresponding to the two parameters passed to the executor of thePromise() constructor.

These properties are defined onPromise.prototype and shared by allPromise instances.

Promise.prototype.constructor

The constructor function that created the instance object. ForPromise instances, the initial value is thePromise constructor.

Promise.prototype[Symbol.toStringTag]

The initial value of the[Symbol.toStringTag] property is the string"Promise". This property is used inObject.prototype.toString().

Promise.prototype.catch()

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.finally()

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

Promise.prototype.then()

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 handleronFulfilled oronRejected is not a function).

In this example, we usesetTimeout(...) to simulate async code.In reality, you will probably be using something like XHR or an HTML API.

const myFirstPromise = new Promise((resolve, reject) => {  // We call resolve(...) when what we were doing asynchronously  // was successful, and reject(...) when it failed.  setTimeout(() => {    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}`);});

This example shows diverse techniques for using Promise capabilities and diverse situations that can occur. To understand this, start by scrolling to the bottom of the code block, and examine the promise chain. Upon provision of an initial promise, a chain of promises can follow. The chain is composed of.then() calls, and typically (but not necessarily) has a single.catch() at the end, optionally followed by.finally(). In this example, the promise chain is initiated by a custom-writtennew Promise() construct; but in actual practice, promise chains more typically start with an API function (written by someone else) that returns a promise.

The example functiontetheredGetNumber() shows that a promise generator will utilizereject() while setting up an asynchronous call, or within the call-back, or both. The functionpromiseGetWord() illustrates how an API function might generate and return a promise in a self-contained manner.

Note that the functiontroubleWithGetNumber() ends with athrow. That is forced because a promise chain goes through all the.then() promises, even after an error, and without thethrow, the error would seem "fixed". This is a hassle, and for this reason, it is common to omitonRejected throughout the chain of.then() promises, and just have a singleonRejected in the finalcatch().

This code can be run under NodeJS. Comprehension is enhanced by seeing the errors actually occur. To force more errors, change thethreshold values.

// To experiment with error handling, "threshold" values cause errors randomlyconst THRESHOLD_A = 8; // can use zero 0 to guarantee errorfunction tetheredGetNumber(resolve, reject) {  setTimeout(() => {    const randomInt = Date.now();    const value = randomInt % 10;    if (value < THRESHOLD_A) {      resolve(value);    } else {      reject(new RangeError(`Too large: ${value}`));    }  }, 500);}function determineParity(value) {  const isOdd = value % 2 === 1;  return { value, isOdd };}function troubleWithGetNumber(reason) {  const err = new Error("Trouble getting number", { cause: reason });  console.error(err);  throw err;}function promiseGetWord(parityInfo) {  return new Promise((resolve, reject) => {    const { value, isOdd } = parityInfo;    if (value >= THRESHOLD_A - 1) {      reject(new RangeError(`Still too large: ${value}`));    } else {      parityInfo.wordEvenOdd = isOdd ? "odd" : "even";      resolve(parityInfo);    }  });}new Promise(tetheredGetNumber)  .then(determineParity, troubleWithGetNumber)  .then(promiseGetWord)  .then((info) => {    console.log(`Got: ${info.value}, ${info.wordEvenOdd}`);    return info;  })  .catch((reason) => {    if (reason.cause) {      console.error("Had previously handled error");    } else {      console.error(`Trouble with promiseGetWord(): ${reason}`);    }  })  .finally((info) => console.log("All done"));

This small example shows the mechanism of aPromise. ThetestPromise() method is called each time the<button> is clicked. It creates a promise that will be fulfilled, usingsetTimeout(), to the promise count (number starting from 1) every 1-3 seconds, at random. ThePromise() constructor is used to create the promise.

The fulfillment of the promise is logged, via a fulfill callback set usingp1.then(). A few logs show how the synchronous part of the method is decoupled from the asynchronous completion of the 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.

HTML

<button>Make a promise!</button><div></div>

JavaScript

"use strict";let promiseCount = 0;function testPromise() {  const thisPromiseCount = ++promiseCount;  const log = document.getElementById("log");  // begin  log.insertAdjacentHTML("beforeend", `${thisPromiseCount}) Started<br>`);  // We make a new promise: we promise a numeric count of this promise,  // starting from 1 (after waiting 3s)  const p1 = new Promise((resolve, reject) => {    // The executor function is called with the ability    // to resolve or reject the promise    log.insertAdjacentHTML(      "beforeend",      `${thisPromiseCount}) Promise constructor<br>`,    );    // This is only an example to create asynchronism    setTimeout(      () => {        // We fulfill the promise        resolve(thisPromiseCount);      },      Math.random() * 2000 + 1000,    );  });  // We define what to do when the promise is resolved with the then() call,  // and what to do when the promise is rejected with the catch() call  p1.then((val) => {    // Log the fulfillment value    log.insertAdjacentHTML("beforeend", `${val}) Promise fulfilled<br>`);  }).catch((reason) => {    // Log the rejection reason    console.log(`Handle rejected promise (${reason}) here.`);  });  // end  log.insertAdjacentHTML("beforeend", `${thisPromiseCount}) Promise made<br>`);}const btn = document.getElementById("make-promise");btn.addEventListener("click", testPromise);

Result

Another example usingPromise andXMLHttpRequest to load an image is shown below.Each step is commented on and allows you to follow the Promise and XHR architecture closely.

<h1>Promise example</h1>

function imgLoad(url) {  // Create new promise with the Promise() constructor;  // This has as its argument a function with two parameters, resolve and reject  return new Promise((resolve, reject) => {    // XHR to load an image    const request = new XMLHttpRequest();    request.open("GET", url);    request.responseType = "blob";    // When the request loads, check whether it was successful    request.onload = () => {      if (request.status === 200) {        // If successful, resolve the promise by passing back the request response        resolve(request.response);      } else {        // If it fails, reject the promise with an error message        reject(          Error(            `Image didn't load successfully; error code: + ${request.statusText}`,          ),        );      }    };    // Handle network errors    request.onerror = () => reject(new Error("There was a network error."));    // Send the request    request.send();  });}// Get a reference to the body element, and create a new image objectconst body = document.querySelector("body");const myImage = new Image();const imgUrl =  "https://mdn.github.io/shared-assets/images/examples/round-balloon.png";// Call the function with the URL we want to load, then chain the// promise then() method with two callbacksimgLoad(imgUrl).then(  (response) => {    // The first runs when the promise resolves, with the request.response    // specified within the resolve() method.    const imageURL = URL.createObjectURL(response);    myImage.src = imageURL;    body.appendChild(myImage);  },  (error) => {    // The second runs when the promise    // is rejected, and logs the Error specified with the reject() method.    console.log(error);  },);

A settings object is anenvironment that provides additional information when JavaScript code is running. This includes the realm and module map, as well as HTML specific information such as the origin. The incumbent settings object is tracked in order to ensure that the browser knows which one to use for a given piece of user code.

To better picture this, we can take a closer look at how the realm might be an issue. Arealm can be roughly thought of as the global object. What is unique about realms is that they hold all of the necessary information to run JavaScript code. This includes objects likeArray andError. Each settings object has its own "copy" of these and they are not shared. That can cause some unexpected behavior in relation to promises. In order to get around this, we track something called theincumbent settings object. This represents information specific to the context of the user code responsible for a certain function call.

To illustrate this a bit further we can take a look at how an<iframe> embedded in a document communicates with its host. Since all web APIs are aware of the incumbent settings object, the following will work in all browsers:

<!doctype html><iframe></iframe><!-- we have a realm here --><script>  // we have a realm here as well  const bound = frames[0].postMessage.bind(frames[0], "some data", "*");  // bound is a built-in function — there is no user  // code on the stack, so which realm do we use?  setTimeout(bound);  // this still works, because we use the youngest  // realm (the incumbent) on the stack</script>

The same concept applies to promises. If we modify the above example a little bit, we get this:

<!doctype html><iframe></iframe><!-- we have a realm here --><script>  // we have a realm here as well  const bound = frames[0].postMessage.bind(frames[0], "some data", "*");  // bound is a built in function — there is no user  // code on the stack — which realm do we use?  Promise.resolve(undefined).then(bound);  // this still works, because we use the youngest  // realm (the incumbent) on the stack</script>

If we change this so that the<iframe> in the document is listening to post messages, we can observe the effect of the incumbent settings object:

<!-- y.html --><!doctype html><iframe src="x.html"></iframe><script>  const bound = frames[0].postMessage.bind(frames[0], "some data", "*");  Promise.resolve(undefined).then(bound);</script>

<!-- x.html --><!doctype html><script>  window.addEventListener(    "message",    (event) => {      document.querySelector("#text").textContent = "hello";      // this code will only run in browsers that track the incumbent settings object      console.log(event);    },    false,  );</script>

In the above example, the inner text of the<iframe> will be updated only if the incumbent settings object is tracked. This is because without tracking the incumbent, we may end up using the wrong environment to send the message.

• Polyfill ofPromise incore-js
• Using promises guide
• Promises/A+ specification
• JavaScript Promises: an introduction on web.dev (2013)
• Callbacks, Promises, and Coroutines: Asynchronous Programming Patterns in JavaScript slide show by Domenic Denicola (2011)