Three kinds of equal

Loose equality coerces, and the rules are genuinely arcane. You need two of them: null == undefined is true and nothing else is loosely equal to either, which makes x == null a legitimate and readable null-or-undefined check. Everything else, use ===.

Strict equality on objects compares references, not contents. Two arrays with identical elements are never ===. This is not a flaw, it is the only comparison that can be done in constant time, but it means "did this change?" is a question you have to answer structurally, either by never mutating in place or by writing a comparison that knows your shape.

Object.is differs from === in exactly two places: NaN is Object.is-equal to itself, and +0 is not Object.is-equal to -0. Those two rows are why React uses Object.is rather than === for its bail-out check, and why a state value of NaN does not cause an infinite re-render loop.