@ungap/event-target/index.js

An EventTarget polyfill

/*! (c) Andrea Giammarchi - ISC */
var self = this || {};
try {
  self.EventTarget = (new EventTarget).constructor;
} catch(EventTarget) {
  (function (Object, wm) {
    var create = Object.create;
    var defineProperty = Object.defineProperty;
    var proto = EventTarget.prototype;
    define(proto, 'addEventListener', function (type, listener, options) {
      for (var
        secret = wm.get(this),
        listeners = secret[type] || (secret[type] = []),
        i = 0, length = listeners.length; i < length; i++
      ) {
        if (listeners[i].listener === listener)
          return;
      }
      listeners.push({target: this, listener: listener, options: options});
    });
    define(proto, 'dispatchEvent', function (event) {
      var secret = wm.get(this);
      var listeners = secret[event.type];
      if (listeners) {
        define(event, 'target', this);
        define(event, 'currentTarget', this);
        listeners.slice(0).some(dispatch, event);
        delete event.currentTarget;
        delete event.target;
      }
      return true;
    });
    define(proto, 'removeEventListener', function (type, listener) {
      for (var
        secret = wm.get(this),
        /* istanbul ignore next */
        listeners = secret[type] || (secret[type] = []),
        i = 0, length = listeners.length; i < length; i++
      ) {
        if (listeners[i].listener === listener) {
          listeners.splice(i, 1);
          return;
        }
      }
    });
    self.EventTarget = EventTarget;
    function EventTarget() {'use strict';
      wm.set(this, create(null));
    }
    function define(target, name, value) {
      defineProperty(
        target,
        name,
        {
          configurable: true,
          writable: true,
          value: value
        }
      );
    }
    function dispatch(info) {
      var options = info.options;
      if (options && options.once)
        info.target.removeEventListener(this.type, info.listener);
      if (typeof info.listener === 'function')
        info.listener.call(info.target, this);
      else
        info.listener.handleEvent(this);
      return this._stopImmediatePropagationFlag;
    }
  }(Object, new WeakMap));
}