isNan() vs Number.isNaN()

`NaN`

NaN stands for Not a number
It represents a value which is not a valid number
It's the one and only value which is not equals to itself
Data Type of NaN is number
NaN is also one of the falsy values

console.log("abc" / 2); // returns NaN

This above line gives NaN because / is a numerical operator which expects two numbers at both side, but since 'abc' is not a number even though js tries coercion.

Coercion

Coercion refers to converting from one dataType to another dataType. There are two types.

Implicit

Js itself tries to convert the datatype

var x1 = "7" - 6;
// string "7" is converted implicitly by js due to "-" is a numerical operator

Explicit

The datatype is converted manually by us.

var x1 = Number("7") - 6;
// Here "7" was converted to number with external factors

Check for NaN

There are two ways to check for NaN

isNaN()
Number.isNaN()

`isNaN()` vs `Number.isNaN()`

isNaN()
Function converts the argument to a number and returns true if the value is NaN
Number.isNaN()
Function checks the argument type if its number and the value is equal to NaN.

Behind the Scenes

`Value(x)`	`isNaN(x)`	`Number.isNaN(x)`
`NaN`	true	true
`undefined`	true	false
`"42,0"`	true	false
`{}`	true	false
`[]`	false	false
`'123'`	false	false
`'123e'`	true	false
`0`	false	false
`-0`	false	false
`""`	false	false
`" "`	false	false
`parseInt('123e')`	false	false

`isNaN()`

// implementation 1
function isNaN(x) {
  let val = Number(x);
  return val !== val;
}
// implementation 2
const isNaN = (x) => Number(x) !== Number(x);
console.log(isNaN(NaN)); // true
// NaN is not a number so prints
console.log(isNaN("123")); // false
// "123" is converted to 123 which is not NaN
console.log(isNaN(null)); // false
// null is converted to 0 which is not NaN
console.log(isNaN("abc")); // true
// "abc" is converted to number which results in NaN

`Number.IsNaN()`

// implementation 1
const NumberIsNaN = (x) => typeof x === "number" && x !== x;
// implementation 2 - with isNaN
const NumberIsNaN = (x) => typeof x === "number" && isNaN(x);
console.log(NumberIsNaN(NaN)); // true
// type of NaN is number and value is not a number
console.log(NumberIsNaN("123")); // false
// "123" is not number hence fails in the first condition itself

Conclusion

Number.IsNaN() is better use if unnecessary coercion is not required

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