Why await only works in async function in javascript?

问题

Just going through this tutorial, and it baffles me to understand why await only works in async function.

From the tutorial:

As said, await only works inside async function.

From my understanding, async wraps the function return object into a Promise, so the caller can use .then()

async function f() {
  return 1;
}

f().then(alert); // 1

And await just waits for the promise to settle within the async function.

async function f() {

  let promise = new Promise((resolve, reject) => {
    setTimeout(() => resolve("done!"), 1000)
  });

  let result = await promise; // wait till the promise resolves (*)

  alert(result); // "done!"
}

f();

It seems to me their usage are not related, could someone please explain?

回答1:

async and await are both meta keywords that allow asynchronous code to be written in a way that looks synchronous. An async function tells the compiler ahead of time that the function will be returning a Promise and will not have a value resolved right away. To use await and not block the thread async must be used.

async function f() {
    return await fetch('/api/endpoint');
}

is equivalent to

function f() {
    return new Promise((resolve,reject) => {
        return fetch('/api/endpoint')
        .then(resolve);
    });
}

来源：https://stackoverflow.com/questions/49640647/why-await-only-works-in-async-function-in-javascript