Some code I wrote to understand the pattern @jashkenas uses to setup the prototype chain in Backbone and Underscore.

js-prototype-play.js

var _ = require('underscore');

function printPrototypeChain(instance, asArray) {
  var chain = []
  while (instance = Object.getPrototypeOf(instance)) {
    var name = instance.constructor.toString().match(/f.+n (.+)\(/);
    chain.push(name && name[1]? name[1] : "(anonymous function expression)")
  }
  return asArray? chain : chain.join(" -> ")
}

var extend = function(protoProps, staticProps) {
  var parent = this;
  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 && _.has(protoProps, 'constructor')) {
    child = protoProps.constructor;
  } else {
    child = function(){ return parent.apply(this, arguments); };
  }

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

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

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

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

  return child;
};

var Level1 = function Level1() {
  console.log('Level1.constructor');
};
Level1.extend = extend;

var Level2 = Level1.extend({
  constructor: function() {
    Level1.prototype.constructor.call(this);
    console.log('Level2.constructor');
  }
});

var Level3 = Level2.extend({
  constructor: function() {
    Level2.prototype.constructor.call(this);
    console.log('Level3.constructor');
  }
});

var Level4 = Level3.extend({
  constructor: function() {
    Level3.prototype.constructor.call(this);
    console.log('Level4.constructor');
  }
});

var test = new Level4;

console.log(printPrototypeChain(test));

console.log(test instanceof Level2);