function addEventListener( event, callback ) {
      if ( _events.hasOwnProperty( event ) ) {
        _events[event].push( callback );
      } else {
        _events[event] = [ callback ];
      }

      return this;
    }
/**
     * Register an event listener.
     * @param {string} event - The event name to listen for.
     * @param {Function} callback - The function called when the event has fired.
     * @returns {Object} The instance this EventObserver instance is decorating.
     */
Register an event listener.
@param {string} event - The event name to listen for.
@param {Function} callback - The function called when the event has fired.
@returns {Object} The instance this EventObserver instance is decorating.
Register an event listener . @param { string } event - The event name to listen for . @param { Function } callback - The function called when the event has fired . @returns { Object } The instance this EventObserver instance is decorating .
function addEventListener( event, callback ) {
      if ( _events.hasOwnProperty( event ) ) {
        _events[event].push( callback );
      } else {
        _events[event] = [ callback ];
      }
      return this;
    }
function addEventListener ( event , callback ) { if ( _events . hasOwnProperty ( event ) ) { _events [ event ] . push ( callback ) ; } else { _events [ event ] = [ callback ] ; } return this ; }
Register an event listener.
Register an event listener .