Changing constructor in JavaScript

Question

I am trying for a while to switch constructor for a object and I am failing. Continuing code will show example of what I need. Thanks.

    <script type="text/javascript">

    function Me(){

      this.name = "Dejan";
    }

    function You(){

        this.name = "Ivan";
    }


    Me.prototype.constructor = You;
    somebody = new Me();


    alert(somebody.name); // **It gives Dejan, and I am expecting Ivan**

    </script>

score 14 · Answer 1 · edited Nov 01 '20 at 07:41

14

The Me.prototype.constructor property is just a public property of Me.prototype, it is not used in any way in the construction of new objects or resolution of property names.

The only vague interest it has is that intially it references the function that its prototype "belongs" to and that instances of Me inherit it. Because it is a public property that can easily be assigned a new value or shadowed, it is not particularly reliable unless you have tight control over it and the code using it.

edited Nov 01 '20 at 07:41

nielsbot

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answered Jun 30 '11 at 02:41

RobG

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*it is not used in any way in the construction of new objects or resolution of property names.* Actually it *is* used in resolving property names. That's why `somebody.constructor === You`. But you're right that it doesn't seem to be used in the construction of new objects. That appears to be a browser bug, and I propose a polyfill in my answer. – personal_cloud May 13 '22 at 23:35
@personal_cloud—in the context of the OP, `new Me()` doesn't go to `Me.prototype.constructor` to create the instance, it calls `Me` directly. Nor does the *Me.prototype.constructor* object sit on the `[[Prototype]]` chain, so it's not used at all in resolving properties of instances (such as `somebody.name`). – RobG May 14 '22 at 05:32
If `Me.prototype.constructor` is not on the prototype chain, then how come assigning `Me.prototype.constructor = You` causes `(new Me()).constructor == You`? – personal_cloud May 15 '22 at 05:24
The property is on the prototype chain, not the object it references. – RobG May 15 '22 at 12:03

score 5 · Accepted Answer · answered Jun 30 '11 at 02:36

You can't change the constructors of an object, you can however change the 'type' of the object the constructor returns, with (as in your example)

Me.prototype = new You();

which would cause new Me()'s to inherit from the object You. However this.name in the Me() function will overshadow the one inherited from you so do something like this:

function Me(){
    this.name =  this.name || "Dejan";
}
function You(){
    this.name = this.name || "Ivan";
}
Me.prototype = new You();
somebody = new Me();
alert(somebody.name);

Output is "Ivan" to clarify for anyone who does not want to run — Ian Walker-Sperber, Aug 18 '18 at 23:36

score 0 · Answer 3 · answered Jun 30 '11 at 02:00

0

try:

Me.prototype = new You();

Me.prototype.constructor = You;
    somebody = new Me();


    alert(somebody.name);

answered Jun 30 '11 at 02:00

DrStrangeLove

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Didn't work for me in Chrome (prints "Dejan"). What's the theory for why this would work? – personal_cloud May 13 '22 at 17:23

personal_cloud · Answer 4 · 2022-05-13T23:36:37.370

As RobG's answer explains, the browser is broken. Identifying the problem is a great first step. However, while "The browser is broken so fix your program" can solve the problem, "The browser is broken so fix the browser" is more logical. Here's one possible polyfill for new.

function NEW(clas, ...args)
{
  let res = Object.setPrototypeOf({}, clas.prototype);
  res.constructor.apply(res, args);
  return res;
}

This fixes the browser so you don't need to change your prototype. Only a minor change is required, to change new to NEW:

Me.prototype.constructor = You;
somebody = NEW(Me);
alert(somebody.name);

And this produces the desired result ("Ivan").

Changing constructor in JavaScript

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