18

This is an oddity I've seen occasionally in JS - maybe someone can shed light on it.

I do a test for undefined on a variable:

if (x !== 'undefined'){}

or even

if (typeof x !== 'undefined'){}

And the browser still throws an error:

ReferenceError: x is not defined

Even 

if (x) {}

throws the error.

This is a framework-level global variable I am checking for, so possibly something to do with different scopes. (No critiques of global variables - again, its the existence of a framework I'm testing for).

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6
  • 7
    second one - typeof x !== 'undefined' should not throw a ReferenceError. Commented Feb 20, 2011 at 19:51
  • 2
    Which browser/version are you working on ? Commented Feb 20, 2011 at 19:55
  • 1
    Anurag: exactly - shouldn't but does Commented Feb 20, 2011 at 20:45
  • Not possible: all major browsers support this correctly and never throw an error for typeof x for any x. Commented Feb 20, 2011 at 22:09
  • can you post a reproducible example on jsfiddle? also what is the configuration (browser/version/os) that it's occurring on if it makes a difference. Commented Feb 21, 2011 at 7:36

3 Answers 3

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12

That's pretty weird. What about:

if (window['x']) {
   // It's defined
}

Does the above work? Also, what browser or JavaScript interpreter is this?

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3 Comments

The problem appears to be crossbrowser (IE/FF/Safari). Using the window['x'] actually works, I can test for !window['x'] without error. Somehow, the browser is testing x in one scope and erroring out in another.
Want to add, even this method not worked, but window.x worked -_- , really weird.
See also the accepted answer here: "You'll have to define it, to be able to check it for a value." Maybe that should read: You'll have to declare it, to be able to check it for a value. if(variable) actually checks for a value and also works to check for possibly non-existing properties: if(obj.variable). If a 'naked' variable might not be declared, its existance can safely be checked with if (typeof(variable) == "undefined").
6

@Anurag is right the second condition should not through error.

if (typeof x !== 'undefined'){// won't through error even if x is not declared
}

I guess the problem is that x is not declared as a variable. If you declare it but leave it unassigned it is treated as undefined and won't through error, in your case it is not declared.

var x;
if (x !== undefined){// won't through error
}

Objects fields are treated differently

if(window.x !== undefined){// won't through error

}

it seems like x in this case is declared in runtime if not found, so returns undefined

Hope this helps!

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2 Comments

I guess the important part here is the window.x !==undefined which i always use. It is okay to check for undefined in the window object, since they always are "declared" when someone checks for them... But that is not applicable to a normal scope though.
@Miguel Overuse of that approach(3rd) will spoil the app with global variables which is a bad practice.
1

Your problem is that undefined !== 'undefined'

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2 Comments

But typeof x === 'undefined' ;)
Fast typing response error xD. I see your point now. As @anurag and @timDown said in FF 20.0.1 you don't get errors with typeof x !== 'undefined'. Good luck testing other browsers

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