When to mark function as async

Question

Basically, function must be prefixed with async keyword if await used inside it. But if some function just returns Promise and doesn't awaiting for anything, should I mark the function as async?

Seems like both correct or not?

// with async (returns Promise)
async getActiveQueue() {
   return redisClient.zrangeAsync(activeQueue, 0, -1);
}

// difference? Both could be awaited isn't it?
getActiveQueue() {
   return redisClient.zrangeAsync(activeQueue, 0, -1);
}

score 24 · Accepted Answer · answered May 22 '17 at 13:22

24

if some function just returns Promise and doesn't awaiting for anything, should I mark the function as async?

I would say you shouldn't. The purpose of async/await is to create (and resolve) the promise for you; if you already have a promise to return, then async/await won't give you any benefit for that function.

Both could be awaited isn't it?

await works on promises, not functions. So, await works fine on any promise, regardless of whether that promise is manually created or created behind the scenes by async.

answered May 22 '17 at 13:22

Stephen Cleary

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5

Using async changes the behaviour of the function for exception handling – Eloims Mar 20 '18 at 10:12
@Eloims: Indeed! If there is a possibility of a synchronous exception being thrown, I would recommend using async, which would capture the exception and resolve the promise with it. – Stephen Cleary Mar 21 '18 at 01:44
4

Shouldn't it be better for clarity, to mark any function that executes asynchronous operation as async? – Qwerty Nov 20 '19 at 13:39
@Qwerty: In my code, I prefer a naming convention - an `Async` suffix. – Stephen Cleary Nov 20 '19 at 15:31

score 12 · Answer 2 · edited Nov 15 '21 at 22:34

The async keyword is not only about allowing the await keyword in the function. It ensures that your function will always return a Promise, even when an exception is raised.

Here are code snippets showing the difference:

async function f(x) {
    if (!x)
        throw new Error('failed')

    return new Promise((resolve, reject) => {
        resolve(x + 1);
    });
}

f(false).then(
    result => console.log('f result', result),
    error => console.log('f async error', error)
);

If you don't add the async keyword and your function raises an exception, it will not be converted to a promise and you'll have to catch it synchronously.

function g(x) {
    if (!x)
        throw new Error('failed');

    return new Promise((resolve, reject) => {
        resolve(x + 1);
    });
}

try {
    g(false).then(
        result => console.log('g result', result),
    )
}
catch (error) {
    console.log('g sync error', error);
}

Marinos An · Answer 3 · 2021-11-16T11:10:42.187

If the function that you call for some unknown reason throws an error, async keyword will make sure that this will be returned as a rejected promise by your function.

async keyword might also be used in functions that would wish to return a promise (e.g. for api consistency) just from the return value, without the need to manually create a Promise object.

Due to the above, I would say that async keyword is not always paired with await.

score 0 · Answer 4 · answered May 22 '17 at 12:37

0

If your function "doesn't awaiting for anything" - this is usual function, even inside your return function may be async ( this is incapsulation)...

answered May 22 '17 at 12:37

Aleksandr Zolotov

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When to mark function as async

4 Answers4