How can I determine if a JavaScript variable is defined in a page?
JavascriptJavascript Problem Overview
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 Solutions
Solution 1 - Javascript
I got it to work using if (typeof(x) != "undefined")
Solution 2 - Javascript
To avoid accidental assignment, I make a habit of reversing the order of the conditional expression:
if ('undefined' !== typeof x) {
Solution 3 - Javascript
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 - Javascript
You can do that with:
if (window.x !== undefined) { // You code here }
Solution 5 - Javascript
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.
Solution 6 - Javascript
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();
Solution 7 - Javascript
try to use undefined
if (x !== undefined)
This is how checks for specific Browser features are done.