Why use object.prototype.constructor in OOP javascript?

Question

I've recently started reading up on OOP javascript and one thing that authors seem to skip over is when an object A has been declared and suddenly I see "A.prototype.constructor =A; For example,

var A = function(){}; // This is the constructor of "A" A.prototype.constructor = A; A.prototype.value = 1; A.prototype.test = function() { alert(this.value); } var a = new A(); // create an instance of A alert(a.value);  // => 1 

So I run the command in firebug "var A = function(){};" and then "A.Constructor" Which reveals it's a function. I understand this.
I run the code "A.prototype.constructor = A;" and I thought this changes the A constructor from Function to A.

The constructor property of A has been changed right? Instead when I run "A.constructor" it gives me function () still.

What's the point?

I also see A.constructor.prototype.constructor.prototype.. what is going on?

Answer 1

If A inherit B using A.prototype = new B();, you need to reset the constructor property for the class A using A.prototype.constructor=A;, otherwise instances of A would have a constructor of B.

In your case, A.prototype.constructor === A will return true, so A.prototype.constructor = A did nothing.

Answer 2

You can quickly test out that that additional assignment does absolutely nothing:

var A = function() {}; A.prototype.constructor === A; // true -- why assign then? 

Resetting the constructor property only makes sense if you've assigned a new prototype object to the class, overwriting the original constructor:

var A = function() {}; A.prototype = protoObject; // some object with members that you'd like to inherit A.prototype.constructor = A; // reset constructor 

In your case, the author might be blindly doing this as good practice, even in cases where it's not necessary.