Javascript 承诺：fetch.then 检索 [[PromiseValue]]

Question

I'm trying to retrieve the [[PromiseValue]] of a Promise. My function currently returns a Promise instead and the value I want myFunction to return is the value stored in [[PromiseValue]] of the returned Promise.

This returns a Promise.

myFunction(){
    return fetch("/api")
        .then(res => {
            return res.json();
        })
        .then(json => {
            return json;
        })
}

I tried this code, but when I print the data in console, it prints the correct value, but the returned value is undefined.

myFunction(){
    return fetch("/api")
        .then(res => {
            return res.json();
        })
        .then(json => {
            return json;
        })
        .then(data => {
            const stringData = data.toString();
            console.log(stringData); // prints the correct string
            return stringData; // returns undefined
        })
}

How would I make my function return the value stored in [[PromiseValue]] as a String? Please help me out, thanks!

Answer 1

Your function cannot return the PromiseValue directly, since fetch works asynchronously. It will return a Promise which will eventually resolve to that value.

Using async/await , what you could do is:

async function myFunction() {
  const res = await fetch('/api');
  const json = await res.json();
  return JSON.stringify(json);
  // json.toString() is a bit weird … but do as you please
  // I'd return the json and parse it at the callsite.
}

const result = await myFunction();

(note: this snippet requires a modern engine which supports top-level await . Latest chrome will work just fine.)