NaN Is A Number

Not-a-Number reports its own type as number, and it never equals itself.

In 2026: Still works. NaN is a value of the Number type by the IEEE 754 standard, so typeof NaN is "number". It is also the only value in the language not equal to itself: NaN === NaN is false. That is exactly how isNaN and Number.isNaN have to test for it.

Where it came from: Both facts come from IEEE 754: NaN is a number-typed value, and the standard defines it as unordered, so it compares unequal to everything, itself included. Wikipedia ↗

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