Here are some reliable testing scenarios for a javascript variable:

var x;

if (x == null) alert("x is undefined");
else if (x === false) alert("x is false");
else if (x === true) alert("x is true");
else alert("x is " + x);

Remarks:
Declare your variable in the beginning of your script if you know you're gonna test it later. Don't give it a value at this point.
A declared variable that has not been assigned a value will have the value 'null'

Use the === compare operator to check whether the variable is really boolean false.
The == operator is not reliable as a value of 0 will also evaluate to false.
(Basically, the === asks whether variable are of same value AND datatype!)

Hope this helps!
Cheers, Jakob

thanks, Jakob.
What if it is not even declared?

i had a look at this page: http://jehiah.cz/archive/javascript-isde...
but it confuses me more than helps. I didn't test the solutions, but according to the comments they don't seem to work in all the main browsers...

Yeah, I wasn't able to find a solution so far for the case that it is not even declared - any testing on a non-declared variable gives a javascript error "x is not defined".

Instead of an individual variable you could also try working with a property of a declared object. That way you don't have to declare the specific property first. You do have to declare the object though, but that could be used for multiple purposes.

Keep me posted if you come across a way of checking wether a variable has been declared.

Jakob

I just checked the link you posted and found this easy solution:

if (window.x === undefined) alert("x is undefined");

Just add the "window." in front of the variable name, then you won't get the error message and the expression will evaluate to true in both cases;
- if the variable has been declared but not assigned
- if the variable has simply not been declared.

Actually it's related to what I suggested about using a property instead of a variable. Since all variables are also properties of the window object!

Jakob

thanks, jgivoni.
points well deserved...

Here is an example (testing the variable "x"):

if (window.x === undefined) alert("x is undefined/undeclared");
else if (x === false) alert("x is false");
else if (x === true) alert("x is true");
else alert("x is " + x);

Notes:

In the first line I am using "window.x" instead of just "x". This works because all variables are also defines as properties of the window object.
"x === undefined" will only work if the variable has actually been declared with "var x" (but not assigned a value). Therefore it is safer to use "window.x" which will not result in an error if the variable hasn't been declared.

Use the === compare operator to check whether the variable is really boolean false.
The == operator is not reliable as a value of 0 will also evaluate to false.
(Basically, the === asks whether variable are of same value AND datatype!)

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