How do promises work in JavaScript?

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A promise is basically an object with two methods. One method is for defining what to do, and one is for telling when to do it. It has to be possible to call the two methods in any order, so the object needs to keep track of which one has been called:

var promise = {
  isDone: false,
  doneHandler: null,
  done: function(f) {
    if (this.isDone) {
        f();
    } else {
        this.doneHandler = f;
    }
  },
  callDone: function() {
    if (this.doneHandler != null) {
        this.doneHandler();
    } else {
        this.isDone = true;
    }
  }
};

You can define the action first, then trigger it:

promise.done(function(){ alert('done'); });
promise.callDone();

You can trigger the action first, then define it:

promise.callDone();
promise.done(function(){ alert('done'); });

Demo: http://jsfiddle.net/EvN9P/

When you use a promise in an asynchronous function, the function creates the empty promise, keeps a reference to it, and also returns the reference. The code that handles the asynchronous response will trigger the action in the promise, and the code calling the asynchronous function will define the action.

As either of those can happen in any order, the code calling the asynchronous function can hang on to the promise and define the action any time it wants.

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