using window as a prototype returns seemingly wrong values in javascript

Question

you'd expect this code to return "123", but instead it returns the window object

function W() {
    this.window = "123";
}

W.prototype = window;

(new W()).window; // window object, not "123"

please check followup question (window as prototype makes setTimeout behave oddly)

Answer 1

The window property on a global window object refers to itself, and it is immutable.

Therefore, the constructor W cannot set the property to 123.

window.window; // returns window object

window.window = "123";

window.window; // still returns window object

If you try to set a non-immutable property in your W constructor, you will see that it works correctly.

function W() {
    this.notWindow = "123";
}

W.prototype = window;

(new W()).notWindow; // returns "123"

This is nothing to do with the prototype, and is instead to do with trying to set immutable properties. Very interesting question, though!

Answer 2

As it was already pointed out window.window is immutable, but you can define own window property in W() constructor function.

function W() {
  Object.defineProperty(this, 'window', {
    configurable: true,
    enumerable: true,
    writable: true,
    value: '123'
  });
}

W.prototype = window;
document.write(new W().window); //123