eventSystem.js

var addEventSystem = function(target){
    // Stores all the events. Keys -> event names, Values -> array of
    // all callbacks for the events
    target.reactionsTo = {};
    target.on = function(event, callback){
        // the event will act as the keys to the system
        this.reactionsTo[event] = this.reactionsTo[event] || []
        this.reactionsTo[event].push(callback);
    }
    // Trigger all the callbacks based on the event being passed
    target.trigger = function(event){
        _.each(this.reactionsTo[event], function(callback){
            callback();
        })
    }
}

// Example
var mouth = {
    eat: function(){}
};
addEventSystem(mouth);

var stomach = {
    digest: function(){}
};
addEventSystem(stomach);

stomach.on('hungry', mouth.eat);
stomach.on('hungry', stomach.digest);