checking for undefined in javascript-- should I use typeof or not?

Question

I'm a bit confused about how best to check if a variable is undefined or not in javascript. I've been doing it like this:

myVar === undefined;

But is it better in all cases to use typeof instead?

typeof myVar === undefined;

And what about the use of undefined vs "undefined", which I've also seen?

Answer 1

This is the best way to check -- totally foolproof:

typeof myVar === "undefined"

This is OK, but it could fail if someone unhelpfully overwrote the global undefined value:

myVar === undefined;

It has to be said that ECMAScript 5 specifies that undefined is read-only, so the above will always be safe in any browser that conforms.

This will never work because it ends up comparing "undefined" === undefined (different types):

typeof myVar === undefined;

Answer 2

This test would always work as expected:

typeof a === 'undefined'

Since the value of undefined can be changed, tests like these aren't always reliable:

a = {}
a.b === undefined

In those cases you could test against void 0 instead:

a.b === void 0
// true

However, this won't work for single variable tests:

a === void 0 // <-- error: cannot find 'a'

You could work around that by testing against window.a, but the first method should be preferred.