Confusion between isNaN and Number.isNaN in javascript

Question

I have a confusion in how NaN works. I have executed isNaN(undefined) it returned true . But if I will use Number.isNaN(undefined) it

Answer 1

• isNaN converts the argument to a Number and returns true if the resulting value is NaN.
• Number.isNaN does not convert the argument; it returns true when the argument is a Number and is NaN.

So which one i should use.

I am guessing you are trying to check if the value is something that looks like a number. In which case the answer is neither. These functions check if the value is an IEEE-754 Not A Number. Period. For example this is clearly wrong:

var your_age = "";
// user forgot to put in their age
if (isNaN(your_age)) {
  alert("Age is invalid. Please enter a valid number.");
} else {
  alert("Your age is " + your_age + ".");
}
// alerts "Your age is ."
// same result when you use Number.isNaN above

Also why there is so discrepancy in the result.

As explained above Number.isNaN will return false immediately if the argument is not a Number while isNaN first converts the value to a Number. This changes the result. Some examples:

                |       Number.isNaN()       |        isNaN()
----------------+----------------------------+-----------------------
value           | value is a Number | result | Number(value) | result
----------------+-------------------+--------+---------------+-------
undefined       | false             | false  | NaN           | true
{}              | false             | false  | NaN           | true
"blabla"        | false             | false  | NaN           | true
new Date("!")   | false             | false  | NaN           | true
new Number(0/0) | false             | false  | NaN           | true