JavaScript: The Definitive Guide, Fourth Edition

Name

isNaN( ) — check for not-a-number

Availability

JavaScript 1.1; JScript 1.0; ECMAScript v1

Synopsis

isNaN(x)

Arguments

x

The value to be tested.

Returns

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

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.

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

See Also

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