The answer to the real question is that you need to do both:

• Setting the prototype to an instance of the parent initializes the prototype chain (inheritance), this is done only once (since the prototype object is shared).


• Calling the parent's constructor initializes the object itself, this is done with every instantiation (you can pass different parameters each time you construct it).

Therefore, you should not call the parent's constructor when setting up inheritance. Only when instantiating an object that inherits from another.

Chris Morgan's answer is almost complete, missing a small detail (constructor property). Let me suggest a method to setup inheritance.

function extend(base, sub) {

  // Avoid instantiating the base class just to setup inheritance

  // Also, do a recursive merge of two prototypes, so we don't overwrite 

  // the existing prototype, but still maintain the inheritance chain

  // Thanks to @ccnokes

  var origProto = sub.prototype;

  sub.prototype = Object.create(base.prototype);

  for (var key in origProto)  {

     sub.prototype[key] = origProto[key];

  }

  // The constructor property was set wrong, let's fix it

  Object.defineProperty(sub.prototype, 'constructor', { 

    enumerable: false, 

    value: sub 

  });

}



// Let's try this

function Animal(name) {

  this.name = name;

}



Animal.prototype = {

  sayMyName: function() {

    console.log(this.getWordsToSay() +   + this.name);

  },

  getWordsToSay: function() {

    // Abstract

  }

}



function Dog(name) {

  // Call the parent's constructor

  Animal.call(this, name);

}



Dog.prototype = {

    getWordsToSay: function(){

      return Ruff Ruff;

    }

}    



// Setup the prototype chain the right way

extend(Animal, Dog);



// Here is where the Dog (and Animal) constructors are called

var dog = new Dog(Lassie);

dog.sayMyName(); // Outputs Ruff Ruff Lassie

console.log(dog instanceof Animal); // true

console.log(dog.constructor); // Dog

See my blog post for even further syntactic sugar when creating classes. http://js-bits.blogspot.com/2010/08/javascript-inheritance-done-right.html

Technique copied from Ext-JS and http://www.uselesspickles.com/class_library/ and a comment from https://stackoverflow.com/users/1397311/ccnokes