isNaN ( number ) The isNaN function is the %isNaN% intrinsic object. When the isNaN function is called with one argument number, the following steps are taken:

1. Let num be ? ToNumber(number).
2. If num is NaN, return true.
3. Otherwise, return false. more info refer doc

Number.isNaN ( number )

1. If Type(number) is not Number, return false.

2. If number is NaN, return true.

3. . Otherwise, return false. More info ref the doc