In browser, assign a new value to "window" object doesn't work?

Question

In browser, I tried to evaluate the following snippet:

window = 1; console.log(window)

The value printed in console is still the original window object instead of number 1.

Does anyone have ideas about why window object can't be re-assigned? Is this a feature of Javascript language? Can I make my own object not writable like window object too?

@StephanBijzitter Is this done in language-level? or in environment-level? Can I make my own object not writable like `window` object too? — Hanfei Sun, Nov 13 '15 at 13:27
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty — epascarello, Nov 13 '15 at 13:30
`Object.defineProperty` allows you to create objects that cannot be overridden. The `window` object is made in a similar fashion, by the engine that runs your code. — Stephan Bijzitter, Nov 13 '15 at 13:30
Yes you can: http://stackoverflow.com/questions/366047/can-read-only-properties-be-implemented-in-pure-javascript — Asons, Nov 13 '15 at 13:30

score 3 · Answer 1 · answered Nov 13 '15 at 13:30

3

It can't be overwritten because it is defined as being non-writable.

That is a feature of JS (although the window object is probably implemented at a lower level).

var example = {};
Object.defineProperty(example, "foo", {
  value: "Hello",
  writable: false
});

document.body.appendChild(document.createTextNode(example.foo));
example.foo = "World";
document.body.appendChild(document.createTextNode(example.foo));

answered Nov 13 '15 at 13:30

Quentin

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This is not a feature of JS but of the DOM API. – bfontaine Nov 13 '15 at 13:32
@bfontaine — The DOM API defines the requirement that it be read only. The JavaScript language implements that requirement. – Quentin Nov 13 '15 at 13:35

score 1 · Accepted Answer · answered Nov 13 '15 at 13:31

The specification says:

The window, frames, and self IDL attributes must all return the Window object's browsing context's WindowProxy object.

The "must" here implying that no matter what you do, every request to window will result in the WindowProxy object.

In browser, assign a new value to "window" object doesn't work?

2 Answers2