for await of VS Promise.all

Question

Is there any difference between this:

const promises = await Promise.all(items.map(e => somethingAsync(e)));
for (c         


        

Answer 1

Yes, they absolutely are different. for await is supposed to be used with asynchronous iterators, not with arrays of pre-existing promises.

Just to make clear,

for await (const res of items.map(e => somethingAsync(e))) …

works the same as

const promises = items.map(e => somethingAsync(e));
for await (const res of promises) …

or

const promises = [somethingAsync(items[0]), somethingAsync(items[1]), …);
for await (const res of promises) …

The somethingAsync calls are happening immediately, all at once, before anything is awaited. Then, they are awaited one after another, which is definitely a problem if any one of them gets rejected: it will cause an unhandled promise rejection error. Using Promise.all is the only viable choice to deal with the array of promises:

for (const res of await Promise.all(promises)) …

See Waiting for more than one concurrent await operation and Any difference between await Promise.all() and multiple await? for details.