Promise API

[jeanpan]

Promise.resolve

let promise = Promise.resolve(value);

Returns a resolved promise with the given value.
Same as

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

The method is used when we already have a value, but would like to have it "wrapped" into a promise.

Example.

function loadCached(url) {
  let cache = loadCached.cache || (loadCached.cache = new Map());

  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;
    });
}

Promise.reject

let promise = Promise.reject(error);

Same as

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

Promise.all
We want to run many promises to execute in parallel, and wait till all of them are ready.

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

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

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 ready, we can show HTTP status codes
    for(let response of responses) {
      alert(`${response.url}: ${response.status}`); // shows 200 for every url
    }

    return responses;
  })
  // map array of responses into 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, Promise.all immediately rejects with that error.

Promise.race
Similar to Promise.all, it takes an iterable of promises, but instead of waiting for all of them to finish, it waits for the first result (or error), and goes on with it.

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