A Singleton in JavaScript that truly ensures one instance, can be lazy-loaded (you can instantiate it whenever you need it), and has private and public scope.

Singleton.js

/* **
 *
 *	Singleton.js - a true Singleton in JavaScript
 *
 *	A Singleton in JavaScript that truly ensures one instance, can be lazy loaded / you 
 *	can instantiate it whenever you need it and has private and public scope.
 *
 *	@author Keith Rosenberg, netPoetica
 *
 * */
var Singleton = function() {
    "use strict";
    
    // If this has has not been instantiated...
    if (!Singleton.instance) {
        console.log("Singleton.instance is undefined. Creating singleton...");

	// Build private members in our private scope. Only the public_members 
	// object will be able to access them. Note: due to the nuances of 
	// JavaScript handling of scope, using the 'this' keyword instead of 
	// 'var' to initialize these members will render them unavailable later.
        var _text = 'private';

        // Build public members object which will be returned in a closure, providing
        // access to the private variables which will no longer be accessible via
        // the Singleton object (i.e. cannot use Singleton._text = 'new text')
        var public_members = {
            text: 'public',
            logText: function() {
            	// Console log the public member variable
                console.info("Singleton>>Public Member: " + this.text);
                
                // Console log the private member variable
                console.info("Singleton>>Private Member: " + _text);
            },
            toString: function() {
            	// Return something more descriptive than [object Object]
                return "[object Singleton]";
            },
            setInstance: function() {
            	// A workaround for passing a reference to the correct 'this' for 
            	// ensuring that the Singleton class cannot be reinstantiated
                Singleton.instance = this;
            }
        };

        // Set the instance as a reference to the public_members object
        public_members.setInstance();
        return public_members;
    }
    else {
        console.warn("Singleton already has an instance, here it is: " + Singleton.instance);
        return Singleton.instance;
    }
};

// Creates a new instance of Singleton
var mySingleton = new Singleton();
mySingleton.logText();

// Recieves the existing instance of Singleton
// herSingleton.logText() is a reference to the same function as mySingleton.logText()
var herSingleton = new Singleton();
herSingleton.logText();