Question

Can anyone explain this code to me

var ParentClass = function(){
}

var ChildClass = function(){
}

//ChildClass.prototype = new ParentClass();

var child = new ChildClass();
alert(child.constructor === ChildClass); // alert true

But

var ParentClass = function(){
}

var ChildClass = function(){
}

ChildClass.prototype = new ParentClass();

var child = new ChildClass();
alert(child.constructor === ChildClass); // alert false

Answer 1

constructor is a property of the prototype object:

var ChildClass = function(){
}

alert(ChildClass.prototype.constructor == ChildClass); // alert true

The relationship is now like this:

  +-------------------+           +--------------------+
  |                   |           |                    |
  |ChildClass instance|---------->|ChildClass protoype |
  |                   |           |  constructor prop  |
  +-------------------+           +--------------------+

This property points indeed to the ChildClass function.

If you override ChildClass.prototype, then child.constructor will be looked up in the prototype chain and will refer to:

ParentClass.prototype.constructor

as ChildClass.prototype is now an instance of ParentClass which inherits from ParentClass.prototype:

  +-------------------+      +--------------------+      +---------------------+
  |                   |      |                    |      |                     |
  |ChildClass instance| ---> |ParentClass instance| ---> |ParentClass prototype|
  |                   |      |                    |      |   constructor  prop |
  +-------------------+      +--------------------+      +---------------------+

ParentClass.prototype.constructor will of course point to ParentClass.