There are 5 static methods in the Promise class. We’ll quickly cover their use cases here.

Promise.all

Let’s say we want to run many promises to execute in parallel, and wait until all of them are ready.

For instance, download several URLs in parallel and process the content when all are done.

That’s what Promise.all is for.

The syntax is:

let promise = Promise.all([...promises...]);

Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise.

The new promise resolves when all listed promises are settled and the array of their results becomes its result.

For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3]:

Promise.all([
  new Promise(resolve => setTimeout(() => resolve(1), 3000)), // 1
  new Promise(resolve => setTimeout(() => resolve(2), 2000)), // 2
  new Promise(resolve => setTimeout(() => resolve(3), 1000))  // 3
]).then(alert); // 1,2,3 when promises are ready: each promise contributes an array member

Please note that the order of the resulting array members is the same as in its source promises. Even though the first promise takes the longest time to resolve, it’s still first in the array of results.

A common trick is to map an array of job data into an array of promises, and then wrap that into Promise.all.

For instance, if we have an array of URLs, we can fetch them all like this:

let urls = [
  'https://api.github.com/users/iliakan',
  'https://api.github.com/users/remy',
  'https://api.github.com/users/jeresig'
];

// map every url to the promise of the fetch
let requests = urls.map(url => fetch(url));

// Promise.all waits until all jobs are resolved
Promise.all(requests)
  .then(responses => responses.forEach(
    response => alert(`${response.url}: ${response.status}`)
  ));

A bigger example with fetching user information for an array of GitHub users by their names (we could fetch an array of goods by their ids, the logic is identical):

let names = ['iliakan', 'remy', 'jeresig'];

let requests = names.map(name => fetch(`https://api.github.com/users/${name}`));

Promise.all(requests)
  .then(responses => {
    // all responses are resolved successfully
    for(let response of responses) {
      alert(`${response.url}: ${response.status}`); // shows 200 for every url
    }

    return responses;
  })
  // map array of responses into an array of response.json() to read their content
  .then(responses => Promise.all(responses.map(r => r.json())))
  // all JSON answers are parsed: "users" is the array of them
  .then(users => users.forEach(user => alert(user.name)));

If any of the promises is rejected, the promise returned by Promise.all immediately rejects with that error.

For instance:

Promise.all([
  new Promise((resolve, reject) => setTimeout(() => resolve(1), 1000)),
  new Promise((resolve, reject) => setTimeout(() => reject(new Error("Whoops!")), 2000)),
  new Promise((resolve, reject) => setTimeout(() => resolve(3), 3000))
]).catch(alert); // Error: Whoops!

Here the second promise rejects in two seconds. That leads to an immediate rejection of Promise.all, so .catch executes: the rejection error becomes the outcome of the whole Promise.all.

In case of an error, other promises are ignored

If one promise rejects, Promise.all immediately rejects, completely forgetting about the other ones in the list. Their results are ignored.

For example, if there are multiple fetch calls, like in the example above, and one fails, other ones will still continue to execute, but Promise.all won’t watch them anymore. They will probably settle, but the result will be ignored.

Promise.all does nothing to cancel them, as there’s no concept of “cancellation” in promises. In another chapter we’ll cover AbortController that can help with that, but it’s not a part of the Promise API.

Promise.all(iterable) allows non-promise “regular” values in iterable

Normally, Promise.all(...) accepts an iterable (in most cases an array) of promises. But if any of those objects is not a promise, it’s passed to the resulting array “as is”.

For instance, here the results are [1, 2, 3]:

Promise.all([
  new Promise((resolve, reject) => {
    setTimeout(() => resolve(1), 1000)
  }),
  2,
  3
]).then(alert); // 1, 2, 3

So we are able to pass ready values to Promise.all where convenient.

Promise.allSettled

A recent addition
This is a recent addition to the language. Old browsers may need polyfills.

Promise.all rejects as a whole if any promise rejects. That’s good for “all or nothing” cases, when we need all results to go on:

Promise.all([
  fetch('/template.html'),
  fetch('/style.css'),
  fetch('/data.json')
]).then(render); // render method needs results of all fetches

Promise.allSettled waits for all promises to settle. The resulting array has:

  • {status:"fulfilled", value:result} for successful responses,
  • {status:"rejected", reason:error} for errors.

For example, we’d like to fetch the information about multiple users. Even if one request fails, we’re still interested in the others.

Let’s use Promise.allSettled:

let urls = [
  'https://api.github.com/users/iliakan',
  'https://api.github.com/users/remy',
  'https://no-such-url'
];

Promise.allSettled(urls.map(url => fetch(url)))
  .then(results => { // (*)
    results.forEach((result, num) => {
      if (result.status == "fulfilled") {
        alert(`${urls[num]}: ${result.value.status}`);
      }
      if (result.status == "rejected") {
        alert(`${urls[num]}: ${result.reason}`);
      }
    });
  });

The results in the line (*) above will be:

[
  {status: 'fulfilled', value: ...response...},
  {status: 'fulfilled', value: ...response...},
  {status: 'rejected', reason: ...error object...}
]

So, for each promise we get its status and value/error.

Polyfill

If the browser doesn’t support Promise.allSettled, it’s easy to polyfill:

if(!Promise.allSettled) {
  Promise.allSettled = function(promises) {
    return Promise.all(promises.map(p => Promise.resolve(p).then(value => ({
      state: 'fulfilled',
      value
    }), reason => ({
      state: 'rejected',
      reason
    }))));
  };
}

In this code, promises.map takes input values, turns them into promises (just in case a non-promise was passed) with p => Promise.resolve(p), and then adds .then handler to every one.

That handler turns a successful result v into {state:'fulfilled', value:v}, and an error r into {state:'rejected', reason:r}. That’s exactly the format of Promise.allSettled.

Now we can use Promise.allSettled to get the results of all given promises, even if some of them reject.

Promise.race

Similar to Promise.all, but waits only for the first settled promise, and gets its result (or error).

The syntax is:

let promise = Promise.race(iterable);

For instance, here the result will be 1:

Promise.race([
  new Promise((resolve, reject) => setTimeout(() => resolve(1), 1000)),
  new Promise((resolve, reject) => setTimeout(() => reject(new Error("Whoops!")), 2000)),
  new Promise((resolve, reject) => setTimeout(() => resolve(3), 3000))
]).then(alert); // 1

The first promise here was fastest, so it became the result. After the first settled promise “wins the race”, all further results/errors are ignored.

Promise.resolve/reject

Methods Promise.resolve and Promise.reject are rarely needed in modern code, because async/await syntax (we’ll cover it a bit later) makes them somewhat obsolete.

We cover them here for completeness, and for those who can’t use async/await for some reason.

  • Promise.resolve(value) creates a resolved promise with the result value.

Same as:

let promise = new Promise(resolve => resolve(value));

The method is used for compatibility, when a function is expected to return a promise.

For example, loadCached function below fetches a URL and remembers (caches) its content. For future calls with the same URL it immediately gets the previous content from cache, but uses Promise.resolve to make a promise of it, so that the returned value is always a promise:

let cache = new Map();

function loadCached(url) {
  if (cache.has(url)) {
    return Promise.resolve(cache.get(url)); // (*)
  }

  return fetch(url)
    .then(response => response.text())
    .then(text => {
      cache.set(url,text);
      return text;
    });
}

We can write loadCached(url).then(…), because the function is guaranteed to return a promise. We can always use .then after loadCached. That’s the purpose of Promise.resolve in the line (*).

Promise.reject

  • Promise.reject(error) creates a rejected promise with error.

Same as:

let promise = new Promise((resolve, reject) => reject(error));

In practice, this method is almost never used.

Summary

There are 5 static methods of Promise class:

  1. Promise.all(promises) – waits for all promises to resolve and returns an array of their results. If any of the given promises rejects, it becomes the error of Promise.all, and all other results are ignored.
  2. Promise.allSettled(promises) (recently added method) – waits for all promises to settle and returns their results as an array of objects with:
    • state: "fulfilled" or "rejected"
    • value (if fulfilled) or reason (if rejected).
  3. Promise.race(promises) – waits for the first promise to settle, and its result/error becomes the outcome.
  4. Promise.resolve(value) – makes a resolved promise with the given value.
  5. Promise.reject(error) – makes a rejected promise with the given error.

Of these five, Promise.all is probably the most common in practice.

Tutorial map

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