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Use the typeof operator to check if a variable is defined or initialized, e.g. if (typeof a !== 'undefined') {} . If the the typeof operator doesn't return a string of "undefined" , then the variable is defined.
So the correct way to test undefined variable or property is using the typeof operator, like this: if(typeof myVar === 'undefined') .
I got it to work using if (typeof(x) != "undefined")
To avoid accidental assignment, I make a habit of reversing the order of the conditional expression:
if ('undefined' !== typeof x) {
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();
}
You can do that with:
if (window.x !== undefined) { // You code here }
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.
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