异步/等待如何工作，为什么在这里不工作

Question

I've been trying to understand how async/await works, all I want to do is have it wait until the value is returned. However, I could not get it to do so with callbacks nor promises nor async/await . What am I doing wrong, and why does async/await not work as I expect it to? (wait with running other code until this promise is resolved)

Many questions like this one link to "introduction pages". I've read them all, I understand what they do, I just don't know how to properly write them out, as I believe I did everything correctly, I'm just missing something console.log("xcdcxcxcxccxvccvffdgfcd") ;

 thing(); async function thing() { let c = await otherthing(); console.log("dfasfadsfdasasdfa" + c) } async function otherthing() { await setTimeout(function() { return new Promise((resolve, reject) => { let f = "rerewwqfewfasgsadf" return resolve(f); }) }, 3000); } 

console.log is supposed to wait until the promised value c is returned, however, it does not seem to work. Why?

Answer 1

Async/await works with functions that return promises. Your otherthing function doesn't return anything.

You can fix the code by returning the promise you are instantiating, like this:

  thing(); async function thing() { let c = await otherthing(); console.log("dfasfadsfdasasdfa" + c) } function otherthing() { return new Promise((resolve, reject) => { setTimeout(function () { let f = "rerewwqfewfasgsadf" resolve(f); }, 3000) }); } 

Answer 2

You must return the new Promise from the otherthing function, not from the setTimeout callback. You are supposed to construct the promise, in its executor callback start the asynchronous action ( setTimeout ) and then asynchronously resolve() or reject() the promise.

function otherthing() {
  return new Promise((resolve, reject) => {
    setTimeout(function(){
      let f = "rerewwqfewfasgsadf"
      resolve(f);
    }, 3000);
  });
}

You don't need any async / await here for the function that is essentially only a promise wrapper around setTimeout . See also How do I convert an existing callback API to promises? .

Answer 3

You should move setTimeout inside Promise body and there do resolve(f)

function otherthing() {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            let f = "rerewwqfewfasgsadf"
            resolve(f);
         }, 3000);
    });
}

Answer 4

async function otherthing has no return statement, so it will always return a promise that resolves to undefined .

await setTimeout(...) doesn't make any sense. You can only await a promise, and setTimeout does not return a promise.

---

You need to explicitly:

• create a new Promise() inside otherthing
• resolve() it with the desired value inside the callback function you pass to setTimeout
• return the promise from otherthing

Don't create a promise inside the callback function you pass to setTimeout . It is pointless there.