JavaScript: The Definitive Guide

24.93. isNaN( ): check for not-a-number

ECMAScript v1

24.93.1. Synopsis

isNaN(x)

24.93.1.1. Arguments

x

The value to be tested.

24.93.1.2. Returns

true if x is (or can be converted to) the special not-a-number value; false if x is any other value.

24.93.2. Description

isNaN( ) tests its argument to determine whether it is the value NaN, which represents an illegal number (such as the result of division by zero). This function is required because comparing a NaN with any value, including itself, always returns false, so it is not possible to test for NaN with the == or === operators.

A common use of isNaN( ) is to test the results of parseFloat( ) and parseInt( ) to determine if they represent legal numbers. You can also use isNaN( ) to check for arithmetic errors, such as division by zero.

24.93.3. Example

isNaN(0); // Returns false isNaN(0/0); // Returns true isNaN(parseInt("3")); // Returns false isNaN(parseInt("hello")); // Returns true isNaN("3"); // Returns false isNaN("hello"); // Returns true isNaN(true); // Returns false isNaN(undefined); // Returns true

24.93.4. See Also

isFinite( ), NaN, Number.NaN, parseFloat( ), parseInt( )