code: function (array, item, index) {
                  if (index == null) {
                      index = array.length;
                  }
                  this.splice(array, index, 0, item);
              }
docs: Add an item to an array, optionally giving the index, and notify listeners. If the index is not provided,
  the item is added at the end of the array.
  @param {Array} array array to be modified
  @param {Object} item item to be added
  @param {Number} index index where to add the item