Javascript inheritance (slight additions to backbone's implementation)

gistfile1.js

// Subclass given parent class and extend with prototype and static properties
// (inherits helper from Backbone.js)
var inherits = function (parent, protoProps, staticProps) {
    var child;

    if (protoProps && protoProps.hasOwnProperty('constructor')) {
        child = protoProps.constructor;
    } else {
        child = function () { parent.apply(this, arguments); };
    }
    _.extend(child, parent);

    var ctor = function () { };
    ctor.prototype = parent.prototype;
    child.prototype = new ctor();

    if (protoProps) _.extend(child.prototype, protoProps);
    if (staticProps) _.extend(child, staticProps);

    child.prototype.constructor = child;
    child.__super__ = parent.prototype;

    return child;
};

// Wrapper for inherits to be applied to Class
var extend = function (constructor, protoProps, classProps) {
    // (This is the primary departure from backbone's implementation)
    // Include constructor argument to make using KnockoutJS's typical
    // viewmodel definition much simpler to use
    if (_.isFunction(constructor)) {
        // Define constructor in prototype properties, if given
        protoProps = protoProps || {};
        protoProps.constructor = constructor;
    } else if (constructor && !classProps) {
        // If constructor is not a function, but exists 
        // and classProps aren't given, 
        // assume no-constructor input is being used
        classProps = protoProps;
        protoProps = constructor;
    }

    var child = inherits(this, protoProps, classProps);
    child.extend = this.extend;
    child.inherit = this.inherit;
    return child;
};

// Convenience for calling parent constructor
var inherit = function (child) {
    this.apply(child, Array.prototype.slice.call(arguments, 1));
    return child;
};

// Extendable method for adding extend and inherit functions to Class
var extendable = function (Class) {
    Class.extend = extend;
    Class.inherit = inherit;
    return Class;
};