Firebase on() does not return anything

Question

I have this piece of code using on() to get data from Firebase, inside on() I create object which I want to send out of function for future use - using return, but it seems it doesn't return anything. So question is how can I make it right?

postsRef.on('value', function(snapshot) {
 if (snapshot.val() === null) {
  var allPosts = false,
  numberOfPosts = 0;
 }
 else {
  var allPosts = snapshot.val(),
  numberOfPosts = Object.size(allPosts);
 }
 var postsData = {
  content: allPosts,
  count: numberOfPosts
 };
 return postsData;
});

Answer 1

The callback function is called asynchronously (some time in the future). So by the time it is invoked, postsRef.on(...) has already returned and any code immediately after it will have run.

For example, this might be tempting, but would not work:

var postsData;
postsRef.on('value', function(snapshot) {
    postsData = snapshot.val();
});
console.log(postsData); // postsData hasn't been set yet!

So there are a few different ways to tackle this. The best answer will depend on preference and code structure:

Move the logic accessing postsData into the callback

postsRef.on('value', function(snapshot) {
    postsData = snapshot.val();
    console.log(postsData);
});

Call another function when the callback is invoked

function logResults(postsData) {
   console.log(postsData);
}

postsRef.on('value', function(snapshot) {
    logResults(snapshot.val());
});

Trigger an event

function Observable() {
   this.listeners = [];
}

Observable.prototype = {
   monitorValue: function( postsRef ) {
      var self = this;
      postsRef.on('value', function(snapshot) {
         self._notifyListeners(postsRef);
      });
   },

   listen:  function( callback ) {
      this.listeners.push(callback);
   },

   _notifyListeners: function(data) {
      this.listeners.forEach(function(cb) {
         cb(data);
      }
   }
};

function logEvent( data ) {
   console.log(data);
}

var observable = new Observable();
observable.listen( logEvent );
observable.monitorValue( /* postsRef goes here */ );