Why does typeof NaN return ‘number’?

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Well, it may seem a little strange that something called “not a number” is considered a number, but NaN is still a numeric type, despite that fact 🙂

NaN just means the specific value cannot be represented within the limitations of the numeric type (although that could be said for all numbers that have to be rounded to fit, but NaN is a special case).

A specific NaN is not considered equal to another NaN because they may be different values. However, NaN is still a number type, just like 2718 or 31415.

As to your updated question to explain in layman’s terms:

A comparison with a NaN always returns an unordered result even when comparing with itself. The comparison predicates are either signalling or non-signalling, the signalling versions signal an invalid exception for such comparisons. The equality and inequality predicates are non-signalling so x = x returning false can be used to test if x is a quiet NaN.

All this means is (broken down into parts):

A comparison with a NaN always returns an unordered result even when comparing with itself.

Basically, a NaN is not equal to any other number, including another NaN, and even including itself.

The comparison predicates are either signalling or non-signalling, the signalling versions signal an invalid exception for such comparisons.

Attempting to do comparison (less than, greater than, and so on) operations between a NaN and another number can either result in an exception being thrown (signalling) or just getting false as the result (non-signalling or quiet).

The equality and inequality predicates are non-signalling so x = x returning false can be used to test if x is a quiet NaN.

Tests for equality (equal to, not equal to) are never signalling so using them will not cause an exception. If you have a regular number x, then x == x will always be true. If x is a NaN, then x == x will always be false. It’s giving you a way to detect NaN easily (quietly).

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