ActionScript: The Definitive Guide

Name

isNaN( ) Global Function — equality test for the special NaN value

Availability

Flash 5

Synopsis

isNaN(value)

Arguments

value

The expression to test.

Returns

A Boolean value: true if value is the special numeric value NaN; otherwise, false.

Description

To test whether or not a value is equal to the special numeric value NaN, we must use the isNaN( ) function because NaN does not test equal to itself in an ordinary equality test. For example, the expression:

NaN == NaN;

yields the value false. The isNaN( ) function is often used to check whether a mathematical error (such as zero divided by itself) has occurred in a phrase of code or whether converting a value to a legitimate number has failed. Because isNaN( ) returns true when the expression is not a valid numeric expression, you’ll often use the logical NOT operator (!) along with isNaN( ) (something that is not not a number is a number). Note that 0/0 yields NaN, but all positive numbers divided by yield Infinity, and all negative numbers divided by yield -Infinity.

Example

// Set a value
var x = "test123";
// Check if x is a legitimate number before using it is in a math expression.
// This is a handy technique for user input in text fields, which always a string.
if (!isNaN(parseFloat(x))) {
  var y = parseFloat(x) * 2;
}

See Also

isFinite( ), NaN; Section 4.3.3 in Chapter 4