code: function addListener(event, callback) {
      checkEventValid(event);
  
      if (listeners[event]) {
          listeners[event].push(callback);
      } else {
          listeners[event] = [callback];
      }
  }
docs: Add a new event listener
  @param {events} event - The name of the event listener to add an event for
  @param {function} callback - The callback to invoke when the event is called