Edit in JSFiddle

(function() {
    function ObserverList() {
        this.observerList = [];
    }

    ObserverList.prototype.add = function(obj) {
        return this.observerList.push(obj);
    };

    ObserverList.prototype.count = function() {
        return this.observerList.length;
    };

    ObserverList.prototype.get = function(index) {
        if (index > -1 && index < this.observerList.length) {
            return this.observerList[index];
        }
    };

    ObserverList.prototype.indexOf = function(obj, startIndex) {
        var i = startIndex;

        while (i < this.observerList.length) {
            if (this.observerList[i] === obj) {
                return i;
            }
            i++;
        }

        return -1;
    };

    ObserverList.prototype.removeAt = function(index) {
        this.observerList.splice(index, 1);
    };

    function Subject() {
        this.observers = new ObserverList();
    }

    Subject.prototype.addObserver = function(observer) {
        this.observers.add(observer);
    };

    Subject.prototype.removeObserver = function(observer) {
        this.observers.removeAt(this.observers.indexOf(observer, 0));
    };

    Subject.prototype.notify = function(context) {
        var observerCount = this.observers.count();
        for (var i = 0; i < observerCount; i++) {
            this.observers.get(i).update(context);
        }
    };
    // The Observer
    function Observer() {
        this.update = function() {
            // ...
        };
    }
    // Extend an object with an extension
    function extend(obj, extension) {
        for (var key in extension) {
            obj[key] = extension[key];
        }
    }

    // References to our DOM elements

    var controlCheckbox = document.getElementById("mainCheckbox"),
        addBtn = document.getElementById("addNewObserver"),
        container = document.getElementById("observersContainer");


    // Concrete Subject

    // Extend the controlling checkbox with the Subject class
    extend(controlCheckbox, new Subject());

    // Clicking the checkbox will trigger notifications to its observers
    controlCheckbox.onclick = function() {
        controlCheckbox.notify(controlCheckbox.checked);
    };

    addBtn.onclick = addNewObserver;

    // Concrete Observer

    function addNewObserver() {

        // Create a new checkbox to be added
        var check = document.createElement("input");
        check.type = "checkbox";

        // Extend the checkbox with the Observer class
        extend(check, new Observer());

        // Override with custom update behaviour
        check.update = function(value) {
            this.checked = value;
        };

        // Add the new observer to our list of observers
        // for our main subject
        controlCheckbox.addObserver(check);

        // Append the item to the container
        container.appendChild(check);
    }

})()

<button id="addNewObserver">Add New Observer checkbox</button>
<input id="mainCheckbox" type="checkbox" />
<div id="observersContainer"></div>