How Can I Determine If A JavaScript Variable Is Defined In A Page?

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.

Solution 1:

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

Solution 2:

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

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

Solution 3:

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();
}

Solution 4:

You can do that with:

if (window.x !== undefined) { // You code here }

Solution 5:

As others have mentioned, the typeof operator can evaluate even an undeclared identifier without throwing an error.

alert (typeof sdgfsdgsd);

Will show "undefined," where something like

alert (sdgfsdgsd);

will throw a ReferenceError.