[Chapter 21] Reference: isNaN()
Author: David Flanagan
Date: Fri Aug 29 11:16:50 1997

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| Chapter 21 JavaScript Reference |

# isNaN() Function

## Name

isNaN() Function---check for Not-a-Number

## Availability

Navigator 3.0; Unix platforms in Navigator 2.0; implemented but nonfunctional in Internet Explorer 3.0

## Synopsis

    isNaN(x)

### Arguments

`x`

  The value to be tested.

### Returns

`true` if `x` is the reserved value `NaN` (not-a-number); `false` if `x` is a legal number, string, or any other type.

## Description

`isNaN()` tests its argument to determine whether it is the reserved 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 `==` operator.

The `NaN` value is not implmemented in Internet Explorer 3.0, so `isNaN()` always returns `false` on that platform.

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

## See Also

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

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