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?

Arboriculture

Why are you trying to make a synchronous operation into an async operation? arr.map() is synchronous and does not return a promise. – Setback

You can't send an asynchronous operation to a function, like map, which expects a synchronous one, and expect it to work. – Blau

@Setback I have many await statements in the inner function. It's actually a long function and I just simplified it to make it readable. I've added now an await call to make it clearer why it should be async. – Arboriculture

You need to await something that returns a promise, not something that returns an array. – Setback

async/await is not part of ES7. – Airla

One useful thing to realise is that every time you mark a function as async, you're making that function return a promise. So of course, a map of async returns an array of promises :) – Weldon

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 probably want to do 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.

Depending on exactly what it is you're trying to do you may also consider using Promise.allSettled, Promise.any, or Promise.race instead of Promise.all, though in most situations (almost certainly including this one) Promise.all will be the one you want.

Matthieu

What do the : colons mean? – Mathura

@DanielPendergast It's for type annotations in TypeScript. – Matthieu

What is the difference between calling callAsynchronousOperation(item); with and without await inside the async map function? – Observe

@Observe That sounds like a good candidate for another question. Basically though, with await the function will wait for the asynchronous operation to complete (or fail) before continuing, otherwise it'll just immediately continue without waiting. – Matthieu

Answer 2

This is simplest way to do it.

await Promise.all(
    arr.map(async (element) => {
        ....
    })
)

Companionable

I keep getting await expressions are only allowed within async functions how does this answer work? – Kantos

@TravisHeeter the function in which you are writing this await expression should be a async function. – Companionable

Answer 3

Solution below to properly use async await and Array.map together. Process all elements of the array in parallel, asynchronously AND preserve the order:

const arr = [1, 2, 3, 4, 5, 6, 7, 8];

const randomDelay = () => new Promise(resolve => setTimeout(resolve, Math.random() * 1000));

const calc = async n => {
  await randomDelay();
  return n * 2;
};

const asyncFunc = async() => {
  const unresolvedPromises = arr.map(calc);
  const results = await Promise.all(unresolvedPromises);
  document.write(results);
};

document.write('calculating...');
asyncFunc();

Also codepen.

Notice we only "await" for Promise.all. We call calc without "await" multiple times, and we collect an array of unresolved promises right away. Then Promise.all waits for resolution of all of them and returns an array with the resolved values in order.

Goble

Over the years of using Javascript, I have always had issues with mapping over arrays without messing up the order due to the async nature of js. I have used promised.all but never thought that it resolves them as is. This answer literally changed how I write maps and other things now. God bless. – Accomplishment

Thanks for this solution, I found it much more eloquent and readable than the others provided. – Azure

Answer 4

There's another solution for it if you are not using native Promises but Bluebird.

You could also try using Promise.map(), mixing the array.map and Promise.all

In you case:

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

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

Jocelyn

It is different - it doesn't run all operations in parallel, but rather executes them in sequence. – Fpc

@AndreyTserkus Promise.mapSeries or Promise.each are sequencial, Promise.map starts them all at once. – Johansen

@AndreyTserkus you can run all or some operations in parallel by providing concurrency option. – Jobye

It's worth mentioning that it's not a vanilla JS. – Serenata

I missed the fact that you are talking about Bluebird when I first read this answer. It would be great to update the code of your answer to explicitly import Promise from Bluebird. – Chancellorsville

Answer 5

If you map to an array of Promises, you can then resolve them all to an array of numbers. See Promise.all.

Servitude

Answer 6

You can use:

for await (let resolvedPromise of arrayOfPromises) {
  console.log(resolvedPromise)
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of

If you wish to use Promise.all() instead you can go for Promise.allSettled() So you can have better control over rejected promises.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled

Turn

Answer 7

I'd recommend using Promise.all as mentioned above, but if you really feel like avoiding that approach, you can do a for or any other loop:

const arr = [1,2,3,4,5];
let resultingArr = [];
for (let i in arr){
  await callAsynchronousOperation(i);
  resultingArr.push(i + 1)
}

FYI: If you want to iterate over items of an array, rather than indices (@ralfoide 's comment), use of instead of in inside let i in arr statement.

Kus

The Promise.all will be async for each element of the array. This will be a sync, it have to wait to finish one element in order to start the next one. – Guadalajara

For those trying this approach, note that for..of is the proper way to iterate an array content, whereas for..in iterates over the indices. – Breadth

Answer 8

A solution using modern-async's map():

import { map } from 'modern-async'

...
const result = await map(myArray, async (v) => {
    ...
})

The advantage of using that library is that you can control the concurrency using mapLimit() or mapSeries().

Deianira

Answer 9

By the way, note that you can easily create an extension to make it cleaner, and use it like this:

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

var results = await arr.mapAsync(async (e)  => {
    return await callAsynchronousOperation(e);
});

You just need to create extensions.ts file:

export {};

declare global {
  interface Array<T> {
    mapAsync<U>(mapFn: (value: T, index: number, array: T[]) => Promise<U>): Promise<U[]>;
}

if (!Array.prototype.mapAsync) {
  Array.prototype.mapAsync = async function <U, T>(
    mapFn: (value: T, index: number, array: T[]) => Promise<U>
  ): Promise<U[]> {
    return await Promise.all(
      (this as T[]).map(async (v, i, a): Promise<U> => {
        return await mapFn(v, i, a);
      })
    );
  };
}

Fibrinogen

Answer 10

I had a task on BE side to find all entities from a repo, and to add a new property url and to return to controller layer. This is how I achieved it (thanks to Ajedi32's response):

async findAll(): Promise<ImageResponse[]> {
    const images = await this.imageRepository.find(); // This is an array of type Image (DB entity)
    const host = this.request.get('host');
    const mappedImages = await Promise.all(images.map(image => ({...image, url: `http://${host}/images/${image.id}`}))); // This is an array of type Object
    return plainToClass(ImageResponse, mappedImages); // Result is an array of type ImageResponse
  }

Note: Image (entity) doesn't have property url, but ImageResponse - has

Hoffman