CODEWITHSUNDEEP
typescript
Gili
Gili

Reputation: 89993

How to use Promise.all() on arrays of different types?

Typescript 4.2.2

Given:

const firstPromises: Promise<First>[] = ...;
const secondPromises: Promise<Second>[] = ...;
return Promise.all([firstPromises, secondPromises]).
  then([first, second] => {
    // do something
  }

I expect first and second to be of type First[] and Second[] respectively. Instead, the output types remain as Promise<First>[] and Promise<Second>[]. Any ideas?

Upvotes: 1

Views: 323

Answers (1)

Alex Wayne
Alex Wayne

Reputation: 186994

You can't pass a nested array of promises to Promise.all. It doesn't know how to handle that, so it just passes it right through to then because it doesn't see any promises. It jsut sees an array, which is doesn't look inside of.

But you can just call Promise.all on each sub array of promises to package that up into a single promise. Then pass each of those to Promise.all to create a promise for both sets.

const getBool = async () => true
const getString = async () => "foo"

function foo() {
    const firstPromises: Promise<boolean>[] = [getBool(), getBool()];
    const secondPromises: Promise<string>[] = [getString(), getString()];
    return Promise.all([
        Promise.all(firstPromises),
        Promise.all(secondPromises)
    ]).then(([first, second]) => {
        // first: boolean[]
        // second: string[]
    })
}

Playground

Upvotes: 1

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