javascript - Are there performance concerns with `return await`?

Question

I see there is an eslint rule, no-return-await, for disallowing return await.

In the rule's description, it states a return await adds "extra time before the overarching Promise resolves or rejects".

However, when I look at MDN async function docs, the "Simple Example" shows an example containing return await without any description of why this might be a performance problem.

Is return await an actual performance problem as the eslint docs suggest?

And if so, how?

Answer 1

No, there isn't any performance problem. It's just an unnecessary extra operation. It might take a bit longer to execute, but should be hardly noticeable. It's akin to return x+0 instead of return x for an integer x. Or rather, exactly equivalent to the pointless .then(x => x).

It doesn't do actual harm, but I'd consider it bad style and a sign that the author does not fully compre­hend promises and async/await.

However, there's one case where it make an important difference:

try {
    …
    return await …;
} …

await does throw on rejections, and in any case awaits the promise resolution before catch or finally handlers are executed. A plain return would have ignored that.