Why does NaN === NaN return false in Javascript?

> undefined === undefined

true

> NaN === NaN

false

> a = NaN

NaN

> a === a

false

On the documentation page I see this:

Testing against NaN

Equality operator (== and ===) cannot be used to test a value against NaN. Use isNaN instead.

Is there any reference that answers to the question? It would be welcome.