backbone's class inheritance isolated to a single function.

extend.js

var extend = (function() {

  var breaker = {};

  var has = function(obj, key) {
    return hasOwnProperty.call(obj, key);
  };

  var each = function(obj, iterator, context) {
    if (obj == null) return;
    if (obj.length === +obj.length) {
      for (var i = 0, l = obj.length; i < l; i++) {
        if (i in obj && iterator.call(context, obj[i], i, obj) === breaker) return;
      }
    } else {
      for (var key in obj) {
        if (has(obj, key)) {
          if (iterator.call(context, obj[key], key, obj) === breaker) return;
        }
      }
    }
  };

  var _extend = function(obj) {
    each(Array.prototype.slice.call(arguments, 1), function(source) {
      for (var prop in source) {
        obj[prop] = source[prop];
      }
    });
    return obj;
  };

  // Shared empty constructor function to aid in prototype-chain creation.
  var ctor = function(){};

  // Helper function to correctly set up the prototype chain, for subclasses.
  // Similar to `goog.inherits`, but uses a hash of prototype properties and
  // class properties to be extended.
  var inherits = function(parent, protoProps, staticProps) {
    var child;

    // The constructor function for the new subclass is either defined by you
    // (the "constructor" property in your `extend` definition), or defaulted
    // by us to simply call the parent's constructor.
    if (protoProps && protoProps.hasOwnProperty('constructor')) {
      child = protoProps.constructor;
    } else {
      child = function(){ parent.apply(this, arguments); };
    }

    // Inherit class (static) properties from parent.
    _extend(child, parent);

    // Set the prototype chain to inherit from `parent`, without calling
    // `parent`'s constructor function.
    ctor.prototype = parent.prototype;
    child.prototype = new ctor();

    // Add prototype properties (instance properties) to the subclass,
    // if supplied.
    if (protoProps) _extend(child.prototype, protoProps);

    // Add static properties to the constructor function, if supplied.
    if (staticProps) _extend(child, staticProps);

    // Correctly set child's `prototype.constructor`.
    child.prototype.constructor = child;

    // Set a convenience property in case the parent's prototype is needed later.
    child.__super__ = parent.prototype;

    return child;
  };

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

})();

// Function.prototype.extend = extend;