Descending sorted array in AngularFire, accomplished by unshift() instead of push().

descending_order_firebasearray.js

app.factory('DescendingArray', function($firebaseArray) {
  return $firebaseArray.$extend({
    
    // Modify $$process to handle added events by prepending instead of appending
    $$process: function(event, rec, prevChild) {
      var key = this.$$getKey(rec);
      var changed = false;
      if( event === 'child_added' ) {
        curPos = this.$indexFor(key);
        changed = this._addBefore(rec, prevChild) !== curPos;
        if( changed ) {
          // send notifications to anybody monitoring $watch
          //todo maybe prevChild here should be the record before?
          //todo we could fetch this with $indexFor(key)-1?
          this.$$notify(event, key, prevChild);
        }
        return changed;  
      }
      else {
        return $firebaseArray.prototype.$$process.apply(this, arguments);
      }
    },
    
    /**
     * Used to insert new records in descending order instead or normal ascending order
     *
     * @param {object} rec
     * @param {string|null} prevChild
     * @private
     */
    _addBefore: function(rec, prevChild) {
      var i;
      if( prevChild === null ) {
        i = this.$list.length;
      }
      else {
        i = this.$indexFor(prevChild);
        if( i === -1 ) { i = 0; }
      }
      this.$list.splice(i, 0, rec);
      this._indexCache[this.$$getKey(rec)] = i;
      return i;
    }
  });
});