Page 164 - JavaScript

checks if the resulting value is NaN. For this reason, this testing method may cause confusion.

(*) The "conversion" method is not that simple, see ECMA-262 18.2.3 for a detailed explanation of the algorithm.

These examples will help you better understand the isNaN() behavior:

isNaN(NaN); // true
isNaN(1); // false: 1 is a number
isNaN(-2e-4); // false: -2e-4 is a number (-0.0002) in scientific notation
isNaN(Infinity); // false: Infinity is a number
isNaN(true); // false: converted to 1, which is a number
isNaN(false); // false: converted to 0, which is a number
isNaN(null); // false: converted to 0, which is a number
isNaN(""); // false: converted to 0, which is a number
isNaN(" "); // false: converted to 0, which is a number
isNaN("45.3"); // false: string representing a number, converted to 45.3
isNaN("1.2e3"); // false: string representing a number, converted to 1.2e3
isNaN("Infinity"); // false: string representing a number, converted to Infinity
isNaN(new Date); // false: Date object, converted to milliseconds since epoch
isNaN("10$"); // true : conversion fails, the dollar sign is not a digit
isNaN("hello"); // true : conversion fails, no digits at all
isNaN(undefined); // true : converted to NaN
isNaN(); // true : converted to NaN (implicitly undefined)
isNaN(function(){}); // true : conversion fails
isNaN({}); // true : conversion fails
isNaN([1, 2]); // true : converted to "1, 2", which can't be converted to a number

This last one is a bit tricky: checking if an Array is NaN. To do this, the Number() constructor first
converts the array to a string, then to a number; this is the reason why isNaN([]) and isNaN([34])
both return false, but isNaN([1, 2]) and isNaN([true]) both return true: because they get converted
to "", "34", "1,2" and "true" respectively. In general, an array is considered NaN by isNaN()
unless it only holds one element whose string representation can be converted to a valid
number.

6

Number.isNaN()

In ECMAScript 6, the Number.isNaN() function has been implemented primarily to avoid the problem
of window.isNaN() of forcefully converting the parameter to a number. Number.isNaN(), indeed,
doesn't try to convert the value to a number before testing. This also means that only values of
the type number, that are also NaN, return true (which basically means only Number.isNaN(NaN)).

From ECMA-262 20.1.2.4:

When the Number.isNaN is called with one argument number, the following steps are
taken:

1. If Type(number) is not Number, return false.
2. If number is NaN, return true.
3. Otherwise, return false.

Some examples:

https://riptutorial.com/ 121