Question

I am trying to create a new class Dog that inherits via prototypical inheritance from the Animal class:

function Animal() {
  this.name = "animal";
  this.writeName = function() {
    document.write(this.name);
  }    
}

function Dog() {
  this.name = "dog";
  this.prototype = new Animal();
}

new Dog().writeName()

​
JS Fiddle

However, I get a Javascript error: Uncaught TypeError: Object #<Dog> has no method 'say'.

Why? Shouldn’t the Dog object retain an Animal object as a prototype?

Answer 1

@ryan’s answer is correct, of course, but he doesn’t really say what’s different about it and it might not be clear to a beginner, so…

The mistake you’re making is that this.prototype = new Animal(); assigns an Animal instance to a property named prototype on the current Dog instance (referred to by this), but there’s nothing special about a property named prototype in this context.

The prototype property is only magical on function objects. When you create a new instance of SomeFunc using new SomeFunc() that new object’s internal/hidden [[prototype]] pointer will refer to the object pointed to by SomeFunc.prototype. The prototype name isn’t special in any other context.