Which answer is correct?

Use `if (x == null)`. This checks for both `null` and `undefined` in one comparison.

— pragmatic_dev

Use `if (typeof x === "undefined")`. This is the safest check — it works even if `x` was never declared.

— safe_coder

Use `if (!x)`. Falsy check covers undefined, null, 0, empty string, and false.

— quick_check

Use `if (x === undefined)`. Direct strict comparison to the `undefined` value.

— strict_mode