extends.js

// JavaScript inheritance with comments

function extends(Derived, Base) {
  // First, we copy all static members from base to derived.
  for (var staticMember in Base) 
    if (Base.hasOwnProperty(staticMember)) 
      Derived[staticMember] = Base[staticMember];

  // Then, we create a tiny helper "class" that constructs derived's prototype, 
  // which just adds the constructor property to its instance.
  DerivedProtoMaker.prototype = Base.prototype;
  function DerivedProtoMaker() { this.constructor = Derived; }
  
  // and finally we make the derived classes prototype. 
  Derived.prototype = new DerivedProtoMaker();
};

// Alternative for the last 3 lines
// Derived.prototype.__proto__ = Base.prototype