Question

Is there anything wrong with this test for undefined?

var undefined;

if(x == undefined){
    //do something
}

or this:

function undefined(x){
    return typeof x == 'undefined';
}

if(undefined(x)){
    //do something
}

jsLint doesn’t throws a reserverd word error, but the code still seems to work…

Answer 1

Don’t redefine undefined.

Other programmers expect undefined to always be undefined, not a function for function’s sake.

People often use typeof operator to ensure a reference error is not thrown when used to test for variables that are undefined.

If anyone ever does this to you, you can use…

undefined = void 0;

… to revert it back.