Return Promise in async function in JavaScript (es2017)

Question

I'm wondering if there is any semantic difference between the 2 functions below in ES2017 or later:

async function returnBool() {
    const promise = new Promise(((res, rej) => {
        setTimeout(() => {
            res(true);
        }, 1000);
    }));
    // returns a boolean
    return await promise;
}

async function returnPromise() {
    const promise = new Promise(((res, rej) => {
        setTimeout(() => {
            res(true);
        }, 1000);
    }));
    // returns a boolean wrapped in a promise
    return promise;
}

I tested both versions on Node and they appear to behave the same. However, I'm looking for some assurances that both versions are syntactically and semantically correct.

In languages such as C#, there is always a discrepancy between the declared return type (Task<TResult>) and the actual return type in the body (TResult) in an async method. Judging from the example above, it seems to be not the case in Javascript? Could this suggest a lack of rigor with JS? What if I want to return a Promise<Promise<T>> from an async method? (unlikely, but still)

It would be really great if anyone could point to some docs or language standard to clarify this. Thanks!

Does this answer your question? [Difference between \`return await promise\` and \`return promise\`](https://stackoverflow.com/questions/38708550/difference-between-return-await-promise-and-return-promise) — joshwilsonvu, Jun 23 '20 at 14:10
`async` functions always return a promise. There is no functional difference between an `async` function and a function returning a promise. The only thing `async` allows you to do is use `await`. So use the second implementation and drop `async`. — Felix Kling, Jun 23 '20 at 14:11
"*What if I want to return a `Promise>` from an async method?*" That is not possible with promises, since they automatically and recursively flatten/join. If you have `new Promise(res1 => res1(new Promise(res2 => res2(42) )))` that simply a single level promise that produces `42` in the end. In many cases, that's fine, but whether it's completely correct [might be up for debate](https://stackoverflow.com/q/45770915). The advantage is some simplicity in chaining. The disadvantage is that sometimes you may want something more advanced. — VLAZ, Jun 23 '20 at 14:37

score 1 · Answer 1 · answered Jun 23 '20 at 14:11

async functions always return a Promise. If you return a value that is not itself a Promise, it will be wrapped in a Promise and that Promise will be returned from the async function.

In second code example, you are explicitly returning a Promise which is unnecessary because, as mentioned above, async functions always return a Promise

Return Promise in async function in JavaScript (es2017)

1 Answers1