Confusion between isNaN and Number.isNaN in javascript

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 is returning false. So which one i should use. Also why there is so discrepancy in the result.

To quote from a ponyfoo article on numbers in ES6:

> Number.isNaN is almost identical to ES5 global isNaN method. > Number.isNaN returns whether the provided value equals NaN. This is a > very different question from “is this not a number?”.

So isNaN just checks whether the passed value is not a number or cannot be converted into a Number. Number.isNaN on the other hand only checks if the value is equal to NaN (it uses a different algorithm than === though).

The String 'ponyfoo' for example is not a number and cannot be converted into a number, but it is not NaN.

Example:

Number.isNaN({});
// <- false, {} is not NaN
Number.isNaN('ponyfoo')
// <- false, 'ponyfoo' is not NaN
Number.isNaN(NaN)
// <- true, NaN is NaN
Number.isNaN('pony'/'foo')
// <- true, 'pony'/'foo' is NaN, NaN is NaN

isNaN({});
// <- true, {} is not a number
isNaN('ponyfoo')
// <- true, 'ponyfoo' is not a number
isNaN(NaN)
// <- true, NaN is not a number
isNaN('pony'/'foo')
// <- true, 'pony'/'foo' is NaN, NaN is not a number

• 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

I found that if you want to check if something is numbery (or not), then a combination of Number.isNaN() with either Number.parseInt() or Number.parseFloat() (depending on what you expect) to cover most use cases:

consider: test a bunch of different input vars against several is number tests:

r = [NaN, undefined, null, false, true, {}, [], '', ' ', 0, 1, '0', '1']
.map(function(v){return [
	v, 
	isNaN(v), 
	Number.isNaN(v), 
    Number.isInteger(v),
	Number.parseInt(v, 10), 
	Number.isNaN( Number.parseInt(v, 10)) 
];});
console.table(r);
// or if console.table() not available:
r.join('\n', function(v){v.join(',')} );

result:

NaN      , true , true , false, NaN, true 
undefined, true , false, false, NaN, true 
null     , false, false, false, NaN, true 
false    , false, false, false, NaN, true 
true     , false, false, false, NaN, true 
Object   , true , false, false, NaN, true 
Array(0) , false, false, false, NaN, true 
''       , false, false, false, NaN, true 
' '      , false, false, false, NaN, true 
0        , false, false, true , 0  , false
1        , false, false, true , 1  , false
'0'      , false, false, false, 0  , false
'1'      , false, false, false, 1  , false

Note the last column, which is usually what I want in my experience.

In shortly,

isNaN()

> will check if the convert of value to Number (type) are failed

For example'abc'

---

Number.isNaN()

> will check if the given value type is Number but not valid number

For example: 'bb'/33

Number.isNaN()

This method returns true if the value is of the type Number, and equates to NaN. Otherwise it returns false. e.g.

Number.isNaN(123) //false

Type of 123 is number but 123 is not equal to NaN, therefore it returns false.


Number.isNaN('123') //false
    
Type of 123 is string & '123' is not equal to NaN, therefore it returns false.


Number.isNaN(NaN) //true

Type of NaN is number & NaN is equal to NaN, therefore it returns true.


Number.isNaN('NaN') //false

Type of 'NaN' is string & 'NaN' is not equal to NaN, therefore it returns false.

global isNaN() function

The global isNaN() function converts the tested value to a Number, then tests it.

isNaN(123) //false
isNaN('123') //false
isNaN(NaN) //true
isNaN('NaN') //true