Javascript – Call async/await functions in parallel

asynchronousbabeljsecmascript-6javascriptnode.js

As far as I understand, in ES7/ES2016 putting multiple await's in code will work similar to chaining .then() with promises, meaning that they will execute one after the other rather than in parallel. So, for example, we have this code:

await someCall();
await anotherCall();

Do I understand it correctly that anotherCall() will be called only when someCall() is completed? What is the most elegant way of calling them in parallel?

I want to use it in Node, so maybe there's a solution with async library?

EDIT: I'm not satisfied with the solution provided in this question: Slowdown due to non-parallel awaiting of promises in async generators, because it uses generators and I'm asking about a more general use case.

Best Answer

You can await on Promise.all():

await Promise.all([someCall(), anotherCall()]);

To store the results:

let [someResult, anotherResult] = await Promise.all([someCall(), anotherCall()]);

Note that Promise.all fails fast, which means that as soon as one of the promises supplied to it rejects, then the entire thing rejects.

const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms))
const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms))

Promise.all([happy('happy', 100), sad('sad', 50)])
  .then(console.log).catch(console.log) // 'sad'

If, instead, you want to wait for all the promises to either fulfill or reject, then you can use Promise.allSettled. Note that Internet Explorer does not natively support this method.

const happy = (v, ms) => new Promise((resolve) => setTimeout(() => resolve(v), ms))
const sad = (v, ms) => new Promise((_, reject) => setTimeout(() => reject(v), ms))

Promise.allSettled([happy('happy', 100), sad('sad', 50)])
  .then(console.log) // [{ "status":"fulfilled", "value":"happy" }, { "status":"rejected", "reason":"sad" }]

Note: If you use Promise.all actions that managed to finish before rejection happen are not rolled back, so you may need to take care of such situation. For example if you have 5 actions, 4 quick, 1 slow and slow rejects. Those 4 actions may be already executed so you may need to roll back. In such situation consider using Promise.allSettled while it will provide exact detail which action failed and which not.

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