coffeescript __extend helper for defining subclass

gistfile1.js

__extends = function(child, parent) { 
  for (var key in parent) { 
    if (__hasProp.call(parent, key)) child[key] = parent[key]; 
  } 
  function ctor() { 
    // why set the prototype's constructor to child?
    this.constructor = child; 
  } 
  ctor.prototype = parent.prototype; 
  child.prototype = new ctor; 
  child.__super__ = parent.prototype; 
  return child; 
};

Is this correct ?

what is __hasProp ?

oh, I know

var __hasProp = {}.hasOwnProperty