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(',')} );
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.