Javascript check if a variable is the window

Question

Hi, do you know a good way to check if a variable is the window object in javascript? I tried with:

var variable=window;
Object.prototype.toString.call(variable);

In Firefox it returns "[object Window]" but in IE "[object Object]" so that's not the right way. Do you know an accurate way to check it?

Answer 1

variable == window

Someone could still define a local variable called window. I'm not sure there's a way that's resilient to all such shenanigans. Someone could make an object that copied most of the window properties and functions, including toString.

Answer 2

if(variable == window)

That of course only checks if the variable object is this window (i.e. the window of the document executing the javascript).

Alternatively, you could try something like this:

if(variable.document && variable.location)

And check for the existance of a few window fields and/or functions so you are pretty sure that it is a window...

Answer 3

How about just:

isWindow = variable === window;

The triple-equals prevents type coercion that otherwise would make this much harder to do.

Answer 4

Yes but i need a way to check every window not only the current one

There are a few ways you can do this. The simplest method is to check for one or two known properties on the window object. There's also the self property - for each window, you could check the self property is equal to the window object:

myvar.self == myvar;
window.self == window;
frameElement.contentWindow.self == frameElement.contentWindow;

Answer 5

Since window is a global variable and global variables are properties of the global object, window.window will equal window. So you could test:

if (mysteryVariable.window == mysteryVariable)
    ...

The problem is that this can be fooled if we have an object like this:

var q = {};
q.window = q;

If that's not likely, then you can use this code.