Are these undefined checking identical in behavior?

Question

I\'ve seem different approaches for (strict equality) checking for undefined:

• if (something === undefined)
• if (typeof some

Answer 1

1. Literally undefined
2. Test for existence, as in "variable not declared".
3. Same as undefined. Older version of the JS standard let you change the value of undefined as it's just a variable. void 0 is undefined, it's safer.

An extra one:

1. if (x == null). Tests for undefined and null because undefined == null but remember, undefined !== null

In JavaScript there's a type 'undefined' and a value undefined. The value undefined is of type 'undefined'.

Answer 2

if (something === undefined) is the standard normal way

typeof something === 'undefined' on a declared variable is mostly an overdefensive solution dating from the time where you could change window.undefined. If you don't know if your variable is declared, it has the advantage of not raising an error but I don't think a legit code should support the case of a variable whose declarative state is unknown.

void 0 (or void anything) is a normalized way to get undefined so it's equivalent to the first one but useless.