Hint
NaN !== NaN always. Number.isNaN() is strict (no coercion). isNaN() converts first.
console.log(NaN === NaN);
console.log(NaN == NaN);
console.log(isNaN('hello'));
console.log(Number.isNaN('hello'));false false true false
Explanation: NaN is the only value not equal to itself. isNaN("hello") coerces to NaN first (true). Number.isNaN doesn't coerce — "hello" is not NaN (false).
Key Insight: NaN !== NaN always. Number.isNaN() is strict (no coercion). isNaN() converts first.