eventemitter.js

/*
  Create an eventEmitter, which has methods "on" and "emit" with the following function signatures.
*/

var EventEmitter = {
    handlers: {}, /* store a list of handler functions against each event */

    /**
    * @param {String} eventName
    * @param {Function} callback
    */ 
    on: function(eventName, callback) {
        /*
            When on is called, add the callback to the list of registered callback for this event type.
        */
        if (this.handlers.hasOwnProperty(eventName)) {
            this.handlers[eventName].push(callback);
        } else {
            this.handlers[eventName] = [callback];
        }
    },

    /**
    * @param {String} eventName
    */

    emit: function(eventName) {
        /*
            Execute all of the callbacks associated with this eventEmitter and eventName.
        */
        if (this.handlers.hasOwnProperty(eventName)) {
            this.handlers[eventName].forEach(function(handler) {
                handler();
            });
        }
    }
};

/*
    Sample usage of the EventEmitter Object
*/

var myEmitter = Object.create(EventEmitter);

myEmitter.on("my-event", function() { 
    console.log("my-event fired, First callback.");
});

myEmitter.on("my-event", function() { 
    console.log("my-event fired, Second callback.");
});

myEmitter.on("your-event", function() { 
    console.log("your-event fired.");
});

myEmitter.emit("your-event");
myEmitter.emit("my-event");