If typeof NaN output 'number' so when is a number not a number?

Question

let a = "hello"*5
value of a is NaN that is short for " Not a Number".
But
typeof a ouput 'number'
Why ?

Answer 1

Spentify understood ! Thanks

Answer 2

A number is never not a number, but a NaN is a number 😁

Answer 3

Sonic 😁😁