javascript prototyping: why doesn't this work?

Question

I'm looking at trying the following code:

var classA = function() {};
classA.prototype = { x: 4, y: 6 };
classA.prototype.prototype = { z: 10 };
var foo = new classA();
alert(foo.z);

Why does the alert come back as undefined? Shouldn't javascript follow the prototype chain to find z?

Answer 1

By default an object prototype is an empty object.

classA.prototype = { x: 4, y: 6 };
classA.prototype.prototype = { z: 10 };

is equivalent to

classA.prototype = { x: 4, y: 6, prototype: { z: 10 }};

you just added a property named prototype to classA

z is a property of the object prototype belonging to classA

alert(foo.prototype.z); will work

Answer 2

I don't believe that you can change the prototype of a prototype or if such a thing even exists. Do this instead

var classA = function() {};
classA.prototype = { x: 4, y: 6 };
classA.prototype.z = 10;
var foo = new classA();
alert(foo.z);