NaN in JavaScript

 window.isNaN()

The global function isNaN() can be used to check if a certain value or expression evaluates to NaN. This function (in

short) first checks if the value is a number, if not tries to convert it (*), and then 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.

Version ≥ 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:

// The one and only

Number.isNaN(NaN); // true

// Numbers

Number.isNaN(1); // false

Number.isNaN(-2e-4); // false

Number.isNaN(Infinity); // false

// Values not of type number

Number.isNaN(true); // false

Number.isNaN(false); // false

Number.isNaN(null); // false

Number.isNaN(""); // false

Number.isNaN(" "); // false

Number.isNaN("45.3"); // false

Number.isNaN("1.2e3"); // false

Number.isNaN("Infinity"); // false

Number.isNaN(new Date); // false

Number.isNaN("10$"); // false

Number.isNaN("hello"); // false

Number.isNaN(undefined); // false

Number.isNaN(); // false

Number.isNaN(function(){}); // false

Number.isNaN({}); // false

Number.isNaN([]); // false

Number.isNaN([1]); // false

Number.isNaN([1, 2]); // false

Number.isNaN([true]); // false