Number.isNaN()

The Number.isNaN() method determines whether the passed value is NaN and its type is Number. It is a more robust version of the original, global isNaN().

Number.isNaN(value)

value

The value to be tested for NaN.

true if the given value is NaN and its type is Number; otherwise, false.

Due to both equality operators, == and ===, evaluating to false when checking if NaN is NaN, the function Number.isNaN() has become necessary. This situation is unlike all other possible value comparisons in JavaScript.

In comparison to the global isNaN() function, Number.isNaN() doesn't suffer the problem of forcefully converting the parameter to a number. This means it is now safe to pass values that would normally convert to NaN, but aren't actually the same value as NaN. This also means that only values of the type number, that are also NaN, return true.

Number.isNaN(NaN);        // true
Number.isNaN(Number.NaN); // true
Number.isNaN(0 / 0);      // true

// e.g. these would have been true with global isNaN()
Number.isNaN('NaN');      // false
Number.isNaN(undefined);  // false
Number.isNaN({});         // false
Number.isNaN('blabla');   // false

// These all return false
Number.isNaN(true);
Number.isNaN(null);
Number.isNaN(37);
Number.isNaN('37');
Number.isNaN('37.37');
Number.isNaN('');
Number.isNaN(' ');

The following works because NaN is the only value in JavaScript which is not equal to itself.

Number.isNaN = Number.isNaN || function isNaN(input) {
    return typeof input === 'number' && input !== input;
}

• Number
• isNaN()