Async/Await inside Array#map()

I'm getting compile time error with 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?

I also tried another way, but it gives me same error:

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

Answers:

Answer

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

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

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.

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