你需要为此使用 Promise.它们在 ES6 中可用，但可以使用 polyfill很容易:

You need to use Promises for this. They are available in ES6 but can be polyfilled quite easily:

function x() {
   var promise = new Promise(function(resolve, reject) {
     window.setTimeout(function() {
       resolve('done!');
     });
   });
   return promise;
}

x().then(function(done) {
  console.log(done); // --> 'done!'
});

在 ES2017 中使用 async/await 如果在 async 函数中会变得更好:

With async/await in ES2017 it becomes nicer if inside an async function:

async function() {
  const result = await x();
  console.log(result); // --> 'done!';
}