gistfile1.js

// If you return a value from the `initialize()` method, that value
// is returned when you instantiate the class using `new`.
// Here, we use this to make `singleton` a class that generates
// new classes. The singleton instance is protected by a closure.

JS.singleton = new JS.Class({
    initialize: function(name, parent, methods) {
        var instance = null;

        return new JS.Class(name, parent, {
            extend: {
                getInstance: function() {
                    return instance = instance || new this();
                }
            },
            initialize: function() {
                if (instance) throw new Error('Cannot reinstantiate a singleton class');
                try { this.callSuper() } catch (e) {}
            },
            include: methods
        });
    }
});
 
// test code
var A = new JS.Class("A", {
    initialize: function(name) {
        this.name = "Albert";
    }
});
 
var B = new JS.singleton("B", A, {
    display: function() {
        console.log("My name is " + this.name);
    }
});
 
var b = B.getInstance(); // runs A.initialize()
var b = B.getInstance(); // does not run A.initialize()
b.display(); // -> "My name is Albert"

B.klass === JS.Class // -> true