Green Green - 6 months ago 4
Javascript Question

Why is it impossible to change constructor function from prototype?

I have such example.

function Rabbit() {
var jumps = "yes";
var rabbit = new Rabbit();
alert(rabbit.jumps); // undefined
alert(Rabbit.prototype.constructor); // outputs exactly the code of the function Rabbit();

I want to change the code in
so that the
var jumps
becomes public. I do it this way:

Rabbit.prototype.constructor = function Rabbit() {
this.jumps = "no";
alert(Rabbit.prototype.constructor); // again outputs the code of function Rabbit() and with new this.jumps = "no";
var rabbit2 = new Rabbit(); // create new object with new constructor
alert(rabbit2.jumps); // but still outputs undefined

Why is it not possible to change the code in constructor function this way?


What is happening is that Rabbit.prototype.constructor is just a pointer to the original constructor (function Rabit(){...}), so that users of the 'class' can detect the constructor from an instance. Therefore, when you try to do

Rabbit.prototype.constructor = function Rabbit() {
    this.jumps = "no";

You're just breaking the link between the original constructor and the reference to it on the prototype. So the answer is, no, you cannot change a constructor by reassigning to prototype.constructor

Inheritance in JS

Many libraries implement inheritance with something like the following:

function extend(base, sub) {

  function surrogateCtor() {}
  // Copy the prototype from the base to setup inheritance
  surrogateCtor.prototype = base.prototype;
  // Tricky huh?
  sub.prototype = new surrogateCtor();
  // The constructor property is set wrong (to the base constructor)
  // with the above trick, let's fix it
  sub.prototype.constructor = sub;

You can see that in the above code, we have to fix the constructor property, but it doesn't affect the actual constructor. See my post about JS inheritance

How to redefine a constructor If you really want to redefine a constructor, just do

var oldProto = Rabbit.prototype;
Rabbit = function() {...};
Rabbit.prototype = oldProto;