Custom EventEmitter for created for the purpose of learning and teaching others.

EventEmitter.js

// Example on codepen https://codepen.io/jagretz/pen/aGeMoJ
// If you would like a production version, use complete, tested, performant, and 
// well-supported version https://github.com/facebook/emitter

/**
 * Subscribe to and control when events are emitted to subscribers.
 */
class EventEmitter {
  constructor() {
    // k,v store of events to subscribers.
    this.events = {};
  }
  /**
   * Register a function (handler) to invoke when the given event (name) is triggered. 
   * @param {string} name to give the event.
   * @param {function} handler to invoke when events are emitted.
   * @return {function} #unsubscribe
   */
  subscribe(name, handler) {
    if (!this.events[name]) this.events[name] = [];
    this.events[name].push(handler);
    
    return this.unsubscribe.bind(this, name, handler);
  }
  /**
   * Remove the handler from the list of subscribers.
   * If the event has no more subscriptions the event is also removed from the list of events.
   */
  unsubscribe(name, handler) {
    console.info(`unsubscribing from ${name}`);
    this.events[name] = this.events[name].filter(fn => handler !== fn);
    if (this.events[name].length <= 0) delete this.events[name];
  }
  /**
   * Identical to #subscribe except that execution only occurs once and immediately unsubscribes.
   * This type of subscription is useful in cases when you only need some behavior to occur
   * one time.
   */
  once(name, handler) {
    /* set the "this" context to a variable to pass into the "once" subscription handler.
     * this is necessary to invoke the unsubscription function when the handler is invoked.
     */
    const self = this;
    /* creates a _new_ subscription handler to control both subscription and unsubscription logic. */
    function onceHandler(...args) {
      self.unsubscribe(name, onceHandler);
      handler.apply(self, args);
    }  
    
    /* subscribe with the _new_ handler. */
    this.subscribe(name, onceHandler);
  }  
  /**
   * Invokes all subscriber functions subscribed to the event.
   * @param {string} name of the event to emit
   * @param {*} data provided to each subscriber.
   */
  emit(name, data) {
    // handler is invoked with .call() to remove the EventEmitter context.
    this.events[name] && this.events[name].forEach(handler => handler.call(null, data));
  }
}