Promise API

There are 6 static methods in the Promise class. We’ll quickly cover their use cases here.

Promise.all

Let’s say we want 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 once they are all done.

That’s what Promise.all is for.

The syntax is:

let promise = Promise.all(iterable);

Promise.all takes an iterable (usually, an array of promises) and returns a new promise.

The new promise resolves when all listed promises are resolved, 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.