为什么需要使用 promise 作为异步回调的包装器

Question

I am new to JS and wanted to understand Promise is JS. So far, I got that Promises are not asynchronous, that is, as you can see in the code below new Promise( function ( ) { console.log( 'b' ); setTimeout( function ( ) { console.log( 'D' ); }, 0 ); } ); When promise object is returned only setTimeOut is executed asychronously and promise serves as a wrapper for that setTimeOut. The question is why need to wrap asynchronous code like setTimeOut inside promise or Ajax requests done with XHR object inside promise.

Answer 1

For sure you don't have to use Promises, you can do everything just with callbacks. But callbacks itself don't really scale well, in the sense that multiple of them don't work together that well.

For example, lets take an asynchronous function that creates a new user account:

  User.create("jonas", result => {
    alert("New account created");
  });

Now, lets create 10 accounts ... Oh right, how does that work? Try it, and you'll see that it is far more complicated. And thats were Promises get relevant. If User.create() would create a promise, we can easily use Promise.all or Promise.race on them, chain them with .then(...) ...

  Promise.all([
    User.create("jonas"),
    User.create("MM")
  ]).then(result => {
    alert("Two users were created");
  });

---

Sidenote: Promises only make sense if you resolve them somewhen, so a good example would be:

  function timer(time) {
    return new Promise( function (resolve) { 
      console.log("promise created synchronously"); 
      setTimeout( function ( ) {
       console.log("callback called asynchronously and promise resolved");
       resolve();
     }, time); 
   });
 }

timer(100).then(/*...*/);