Use async await with Array.map

Question

Given the following code:

var arr = [1,2,3,4,5];

var results: number[] = await arr.map(async (item): Promise<number> => {
        await callAsynchronousOperation(item);
        return item + 1;
    });

which produces the following error:

TS2322: Type 'Promise<number>[]' is not assignable to type 'number[]'. Type 'Promise<number> is not assignable to type 'number'.

How can I fix it? How can I make async await and Array.map work together?

Answer 1

The problem here is that you are trying to await an array of promises rather than a promise. This doesn't do what you expect.

When the object passed to await is not a Promise, await simply returns the value as-is immediately instead of trying to resolve it. So since you passed await an array (of Promise objects) here instead of a Promise, the value returned by await is simply that array, which is of type Promise<number>[].

What you need to do here is call Promise.all on the array returned by map in order to convert it to a single Promise before awaiting it.

According to the MDN docs for Promise.all:

The Promise.all(iterable) method returns a promise that resolves when all of the promises in the iterable argument have resolved, or rejects with the reason of the first passed promise that rejects.

So in your case:

var arr = [1, 2, 3, 4, 5];

var results: number[] = await Promise.all(arr.map(async (item): Promise<number> => {
    await callAsynchronousOperation(item);
    return item + 1;
}));

This will resolve the specific error you are encountering here.