CODEWITHSUNDEEP
javascriptundefined
larryq
larryq

Reputation: 16299

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

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?

Upvotes: 14

Views: 2672

Answers (3)

de3
de3

Reputation: 2000

I believe that in the most common cases, e.g. when checking if a parameter is passed through a function, myVar === undefined is enough, as myVar will be always declared as a parameter

Upvotes: 0

Jon
Jon

Reputation: 437336

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;

Upvotes: 22

Ja͢ck
Ja͢ck

Reputation: 173552

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.

Upvotes: 2

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