JQuery/Javascript: check if var exists [duplicate]

I suspect there are many answers like this on SO but here you go:

if ( typeof pagetype !== 'undefined' && pagetype == 'textpage' ) {
  ...
}

You can use typeof:

if (typeof pagetype === 'undefined') {
    // pagetype doesn't exist
}

For your case, and 99.9% of all others elclanrs answer is correct.

But because undefined is a valid value, if someone were to test for an uninitialized variable

var pagetype; //== undefined
if (typeof pagetype === 'undefined') //true

the only 100% reliable way to determine if a var exists is to catch the exception;

var exists = false;
try { pagetype; exists = true;} catch(e) {}
if (exists && ...) {}

But I would never write it this way

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