JavaScript - check if in global context

Question

When a function is attached to an object and called:

function f() { return this.x; }
var o = {x: 20};
o.func = f;
o.func(); //evaluates to 20

this refers to the object that the function was called as a method of. It's equivalent to doing f.call(o).

When the function is called not as part of an object, this refers to the global object. How do I check if a function is being called from a non-object context? Is there any standard keyword to access the global object? Is the only way to do it something like this?

globalobj = this;
function f() { if (this == globalobj) doSomething(); }

Note: I have no particular use case in mind here - I actually am asking about this exact mechanism.

Mike Samuel · Accepted Answer · 2008-12-22 01:18:53Z

9

The below should work since using Function.call with a value of null will invoke it in the global scope.

this === ((function () { return this; }).call(null))

A simpler variant,

this === (function () { return this; })()

will also work, but I think the first makes the intent clearer.

answered Dec 22, 2008 at 1:18

Mike Samuel

121k30 gold badges230 silver badges255 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

GeekyDeaks Over a year ago

The tags do not specify if this is browser only question and this answer works in both node.js and a browser

Greg · Accepted Answer · 2008-12-20 10:49:41Z

8

The global object is actually the window so you can do

if (this === window)

answered Dec 20, 2008 at 10:49

Greg

323k55 gold badges378 silver badges338 bronze badges

6 Comments

some Over a year ago

True at least if the script is running in a browser (since javascript now can be run in other places it might not be a universal truth)

Claudiu Over a year ago

is this a standard across browsers?

fasih.rana Over a year ago

That depends on the browsers implementation of JavaScript engine, but that is mostly the case.

some Over a year ago

It's true for FF, IE, Opera and Chrome on Windows. A little more info: quirksmode.org/js/this.html

some Over a year ago

"Objects and this", "In a simple function call, this is set to the Global Object (aka window), which is not very useful.", javascript.crockford.com/survey.html

|

olliej · Accepted Answer · 2008-12-21 08:25:17Z

1

RoBorg's answer is conceptually correct -- except window is only available in the context of the browsers main thread (so this necessarily excludes worker threads and the like, as well as any non-browser hosted JS, which is getting less and less uncommon).

Your safest bet is basically what you had above, but you should use var and === as it is possible for the interpreter to optimise such accesses more completely.

answered Dec 21, 2008 at 8:25

olliej

36.9k9 gold badges61 silver badges55 bronze badges

Collectives™ on Stack Overflow

JavaScript - check if in global context

3 Answers 3

1 Comment

6 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related