example of observer pattern.

jsDesignPattern.observer.js

"use strict";
var publisher = {
    subscribers: {
        any: [] // event type: subscribers
    },
    subscribe: function (fn, type) {
        type = type || 'any';
        if (typeof this.subscribers[type] === "undefined") {
            this.subscribers[type] = [];
        }
        this.subscribers[type].push(fn);
    },
    unsubscribe: function (fn, type) {
        this.visitSubscribers('unsubscribe', fn, type);
    },
    publish: function (publication, type) {
        this.visitSubscribers('publish', publication, type);
    },
    visitSubscribers: function (action, arg, type) {
        var pubtype = type || 'any',
            subscribers = this.subscribers[pubtype],
            i,
            max = subscribers.length;
            
        for (i = 0; i < max; i += 1) {
            if (action === 'publish') {
                subscribers[i](arg);
            } else {
                if (subscribers[i] === arg) {
                    subscribers.splice(i, 1);
                }
            }
        }
    }
};

/*
var s1 = {log: console.log},
    s2 = {err: console.error},
    s3 = {warn: console.warn};


publisher.subscribe(s1.log);
publisher.subscribe(s2.err);
publisher.subscribe(s3.warn);

publisher.publish({hello: "World"});

publisher.unsubscribe(s2.err);
publisher.publish("hello");


publisher.subscribe(s1.log, "log");
publisher.publish({obj: "log this object"}, "log");
*/

function makePublisher(o) {
    var i;
    for (i in publisher) {
        if (publisher.hasOwnProperty(i) && typeof publisher[i] === "function") {
            o[i] = publisher[i];
        }
    }
    o.subscribers = {any: []};
}

var paper = {
    daily: function () {
        this.publish("big news today");
    },
    monthly: function () {
        this.publish("interesting analysis", "monthly");
    }
};

makePublisher(paper);

var joe = {
    drinkCoffee: function (paper) {
        console.log('Just read ' + paper);
    },
    sundayPreNap: function (monthly) {
        console.log('About to fall asleep reading this ' + monthly);
    }
};

paper.subscribe(joe.drinkCoffee);
paper.subscribe(joe.sundayPreNap, 'monthly');

paper.daily();
paper.daily();
paper.daily();
paper.monthly();


makePublisher(joe);

joe.tweet = function (msg) {
    this.publish(msg);
};

paper.readTweets = function (tweet) {
    alert('Call big meeting! Someone ' + tweet);
};

joe.subscribe(paper.readTweets);

joe.tweet("hated the paper today");