我可以在没有.then的情况下使用async等待吗？

Question

I'm a bit confused about the MDN documentation of async await . These docs use the .then() syntax to respond when an async function has resolved:

mdn example

async function add() {
  const b = await resolveAfter2Seconds(30); // timer
  return b;
}
add().then(v => {
  console.log(v);  
});

But in my own code, I don't use .then() and it still works asynchronously. The flow is not blocked. So why would you use .then() ?

async code without .then

function start(){
    console.log("starting!")
    let d = loadData()
    console.log("this message logs before loadData returns!")
    console.log(d)   // this shows: Promise - Pending because d has not yet returned
}
async function loadData() {
    const response = await fetch("https://swapi.co/api/films/");
    const json = await response.json();
    console.log("data loaded!")
    return json;
}

Answer 1

First of all, all async functions return a Promise, so if you want to get the returned value from that async operation, you'll either need to use then or await inside an async function.

MDN uses .then because the add async function is being called outside an async function scope, so it can't use await globally to catch the data from the promise.

In your example, you get the same, an instance of Promise as the return of your loadData async function, if you define the start function also as async, you can use let d = await loadData() , if it's not async, you can use .then (which is the Promise API ).

Answer 2

Async function declaration async function loadData() returns AsyncFunction which is executed asynchronously and always returns a Promise .

Basically all the code you put inside the async function someFunctionName will be executed inside that Promise and when you return some value inside that function – it will resolve that promise.

So that then() call is to get the actual value from that Promise that your function returned.

Answer 3

Your code works because it is not returning promise object and actually waiting for the response. It returns the json at the end. So, execution holds till it gets the response.

If your function is async you don't need to return promise, your return statement will wait for all 'await' statement before it to complete.

Answer 4

您自己的代码有效，因为您将.then()的用法替换为async await的用法。