How can I determine if a JavaScript variable is defined in a page?

SSharma_22628 0
Viewed 97651

How can i check in JavaScript if a variable is defined in a page? Suppose I want to check if a variable named "x" is defined in a page, if I do if(x != null), it gives me an error.

javascript
7 Answers

I got it to work using if (typeof(x) != "undefined")

SSharma_226280

To avoid accidental assignment, I make a habit of reversing the order of the conditional expression:

if ('undefined' !== typeof x) {
Andrew_Hedges_115770

The typeof operator, unlike the other operators, doens't throws a ReferenceError exception when used with an undeclared symbol, so its safe to use...

if (typeof a != "undefined") {
    a();
}
Pablo_Cabrera_125400

Assuming your function or variable is defined in the typical "global" (see: window's) scope, I much prefer:

if (window.a != null) {
   a();
}

or even the following, if you're checking for a function's existence:

if (window.a) a();
kamens_13350

try to use undefined

if (x !== undefined)

This is how checks for specific Browser features are done.

user_170
Related