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'.