Why is NaN === NaN false? [duplicate]

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Strict answer: Because the JS spec says so:

  • If Type(x) is Number, then
    • If x is NaN, return false.
    • If y is NaN, return false.

Useful answer: The IEEE 754 spec for floating-point numbers (which is used by all languages for floating-point) says that NaNs are never equal.

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