If you await a Promise.all with an array of the promises, it will take approximately 1000ms.
In summary, using individual awaits runs them serially, while Promise.all runs them concurrently.
If you’re doing CPU bound work without workers, it doesn’t make much of a difference, but if you’re doing I/O bound tasks, like HTTP requests, then doing it in parallel will likely make a significant difference.
https://codepen.io/tomtheisen/pen/QWPOmjp
function delay(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
async function test() {
const start = new Date;
const promises = Array(3).fill(null).map(() => delay(1000));
for (const p of promises) await p;
const end = new Date;
console.log("elapsed", end - start); // shows about 1010
}
test();Whereas if you started one promise, waited for it to finish, then started the next and so on, it would take the three seconds as they won't be run in parallel.
The code you've written can be seen as a "poor-man's" Promise.all, in the sense that it's doing roughly the same thing but less clearly. It also behaves slightly differently in terms of rejections: if the final promise in the Promise.all version rejects immediately, then the whole promise will fail immediately. However, in your version, if the final promise rejects, that rejection won't be evaluated by the await (and therefore thrown) until all the other tasks have completed.
For reasons of clarity and correctness, therefore, it's usually better to just use Promise.all rather than awaiting a list of already-started promises in sequence.
for (var i = 0; i < 3; i++) {
await delay(1000);
}
const promises = Array(3).fill(null).map(() => delay(1000));
Promise.all is just an alternative to writing the for...of await loop:
await Promise.all(promises);
A strong case for preferring the Promise.all() is that Promise.allSettled(), Promise.any() and Promise.race() also exist for working with collections of promises, and unlike Promise.all(), they would not be so easily reproduced with a one liner for...of loop, so its not unreasonable to expect that JS developers should be aware of Promise.all(), meaning there is no reason for it not to be the preferred syntax for the reasons I stated above.