What exactly does isNaN() do in javaScript? [duplicate]

Question

isNaN() returns false if and only if the argument is number.

var a = new Number(1);

By doing so, a is an object now. So why is isNaN(a) returning false?

Answer 1

isNaN() tests whether an input is NOT a number.

var a = new Number(1)

a is a number object. Therefore isNaN() returns false.

Here is a nice documentation containing isNaN() behaviors with different edge cases. Unfortunately some of them are not as intuitive as hoped, so it's worth reading.