simple-pubsub.es6.js

class PubSub {
  constructor() {
    this.handlers = [];
  }

  subscribe(event, handler, context) {
    if (typeof context === 'undefined') {
      context = handler;
    }
    
    this.handlers.push({event: event, handler: handler.bind(context)});
  }

  publish(event) {
    for (let i = 0; i < this.handlers.length; i++) {
      if (this.handlers[i].event === event) {
        this.handlers[i].handler.call();
      }
    }
  }
}

simple-pubsub.js

var PubSub = function() {
  this.handlers = [];
};

PubSub.prototype = {
  constructor: PubSub,

  subscribe: function(event, handler, context) {
    if (typeof context === 'undefined') {
      context = handler;
    }
    this.handlers.push({event: event, handler: handler.bind(context)});
  },

  publish: function(event) {
    for (var i = 0; i < this.handlers.length; i++) {
      if (this.handlers[i].event === event) {
        this.handlers[i].handler.call();
      }
    }
  }
};

What about when you need to pass arguments?