Namespacing Event Listener in Vanilla Javascript (:see_no_evil: Monkey Patch)

event.namespace.js

(function () {
  'use strict';

  EventTarget.prototype.on = function (event, listener, options) {
    if (arguments.length < 2)
      throw new TypeError("Failed to execute 'on' on 'EventTarget': 2 arguments required, but only " + arguments.length + " present.");

    var typeOfListener = typeof listener;
    if (typeOfListener !== 'function' && typeOfListener !== 'object')
      throw new TypeError("Failed to execute 'on' on 'EventTarget': The callback provided as parameter 2 is not an object.");

    if (!this.hasOwnProperty('listeners'))
      Object.defineProperty(this, 'listeners', {
        value: [],
        configurable: false,
        enumerable: true,
        writable: false
      });

    event = String(event).trim();
    var eventStruct = event.split('.');

    this.listeners.push({event: event, listener: listener});
    this.addEventListener(eventStruct[0], listener, options);
    return this;
  };

  EventTarget.prototype.off = function (event, options) {
    if (arguments.length < 1)
      throw new TypeError("Failed to execute 'off' on 'EventTarget': 1 arguments required, but only " + arguments.length + " present.");

    if (!this.hasOwnProperty('listeners')) return this;

    event = String(event).trim();
    var eventStruct = event.split('.');
    var type = eventStruct[0];
    var namespace = eventStruct[1];
    var isTypeOnly = eventStruct.length === 1;
    var isNamespaceOnly = eventStruct.length > 1 && type.length === 0;

    var _idx = this.listeners.length;
    while (_idx--) {
      var _obj = this.listeners[_idx], _event = _obj.event, _listener = _obj.listener;
      if (event === _event) {
        this.removeEventListener(type, _listener, options);
        this.listeners.splice(_idx, 1);
      } else if (isTypeOnly || isNamespaceOnly) {
        var _eventStruct = _event.split('.'), _type = _eventStruct[0], _namespace = _eventStruct[1];
        if (isTypeOnly
          ? (type !== _type)
          : (namespace !== _namespace)) continue;
        this.removeEventListener(_type, _listener, options);
        this.listeners.splice(_idx, 1);
      }
    }

    return this;
  };
})();

for Typescript

// index.d.ts

interface EventTarget {
  listeners?: {
    event: string,
    listener: EventListenerOrEventListenerObject
  }[];
  on(event: string, listener: EventListenerOrEventListenerObject | null, options?: boolean | AddEventListenerOptions): this;
  off(event: string, options?: EventListenerOptions | boolean): this;
}