async/await inside arrow functions (Array#map/filter)

Question

I'm getting compile time error in this code:

const someFunction = async (myArray) => {
    return myArray.map(myValue => {
        return {
            id: "my_id",
            myValue: await service.getByValue(myValue);
        }
    });
};

Error message is:

await is a reserved word

Why can't I use it like this?

Answer 1

You can't do this as you imagine, because you can't use await if it is not directly inside an async function.

The sensible thing to do here would be to make the function passed to map asynchronous. This means that map would return an array of promises. We can then use Promise.all to get the result when all the promises return. As Promise.all itself returns a promise, the outer function does not need to be async.

const someFunction = (myArray) => {
    const promises = myArray.map(async (myValue) => {
        return {
            id: "my_id",
            myValue: await service.getByValue(myValue)
        }
    });
    return Promise.all(promises);
}

Answer 2

If you want to run map with an asynchronous mapping function you can use the following code:

const resultArray = await Promise.all(inputArray.map(async (i) => someAsyncFunction(i)));

How it works:

• inputArray.map(async ...) returns an array of promises - one for each value in inputArray.
• Putting Promise.all() around the array of promises converts it into a single promise.
• The single promise from Promise.all() returns an array of values - the individual promises each resolve to one value.
• We put await in front of Promise.all() so that we wait for the combined promise to resolve and store the array of resolved sub-promises into the variable resultArray.

In the end we get one output value in resultArray for each item in inputArray, mapped through the function someAsyncFunction. We have to wait for all async functions to resolve before the result is available.

Answer 3

That's because the function in map isn't async, so you can't have await in it's return statement. It compiles with this modification:

const someFunction = async (myArray) => {
    return myArray.map(async (myValue) => { // <-- note the `async` on this line
        return {
            id: "my_id",
            myValue: await service.getByValue(myValue)
        }
    });
};

Try it out in Babel REPL

So… it's not possible to give recommendation without seeing the rest of your app, but depending on what are you trying to do, either make the inner function asynchronous or try to come up with some different architecture for this block.

Update: we might get top-level await one day: https://github.com/MylesBorins/proposal-top-level-await

Answer 4

it will be 2 instructions, but just shift "await" with the extra instruction

let results = array.map((e) => fetch('....'))
results  = await Promise.all(results)

Answer 5

I tried all these answers but no one works for my case because all answers return a promise object not the result of the promise like this:

{
  [[Prototype]]: Promise
  [[PromiseState]]: "fulfilled"
  [[PromiseResult]]: Array(3)
  0: ...an object data here...
  1: ...an object data here...
  2: ...an object data here...
  length: 3
  [[Prototype]]: Array(0)
}

Then I found this answer https://stackoverflow.com/a/64978715/8339172 that states if map function is not async or promise aware. So instead of using await inside map function, I use for loop and await the individual item because he said that for loop is async aware and will pause the loop.

Answer 6

When you want each remapped value resolved before moving on to the next, you can process the array as an asynchronous iterable.

Below, we use library iter-ops, to remap each value into promise, and then produce an object with resolved value, because map itself shouldn't be handling any promises internally.

import {pipe, map, wait, toAsync} from 'iter-ops';

const i = pipe(
    toAsync(myArray), // make asynchronous
    map(myValue => {
        return service.getByValue(myValue).then(a => ({id: 'my_id', myValue: a}))
    }),
    wait() // wait for each promise
);

(async function() {
    for await (const a of i) {
        console.log(a); // print resulting objects
    }
})

After each value, we use wait to resolve each remapped value as it is generated, to keep resolution requirement consistent with the original question.