# Using The isNaN() Function in JavaScript, With Examples

isNaN() is a JavaScript function that will tell you whether a value is equal to NaN – or Not a Number. It can be used to determine whether the result of a mathematical operation is valid or not. Here’s how to use it.

## What is NaN?

• NaN is a special value that means Not a Number.
• It means that a value that should be a number could not be parsed as a number.
• Any mathematical operation between any other value and NaN will result in NaN.
• It usually means something has gone wrong with a number parsing or mathematical operation and that you need to check your code or handle input differently.

NaN was also explored in our article on the JavaScript parseInt() function.

## JavaScript isNaN() Function Syntax

The isNaN() function will check whether a value or variable has a value equal to NaN when JavaScript tries to parse it as a number.

Here’s the syntax for the isNaN() function in JavaScript:

`isNaN(VALUE)`

Note that:

• VALUE is a value, or variable, to check
• isNaN() will return a boolean value (TRUE or FALSE)
• It will return TRUE if VALUE is equal to NaN after JavaScript has attempted to parse it as a number and FALSE if otherwise

## Examples

```var myNumber = 4.32;

var myString = "foo";

var myNumberString = "1234";

var myNaN = NaN;

console.log(isNaN(undefined)); // true - undefined evaluates as NaN

console.log(isNaN(myNumber)); // false - 4.32 evaluates as a number

console.log(isNaN(myString)); // true - the given string "foo" cannot be parsed as a number

console.log(isNaN(myNumberString)); // false - "1234" is a string which can be parsed as a number

console.log(isNaN(myNumber * myString)); // true - the result of multiplying 4.32 and "foo" could not be calculated as a number

console.log(isNaN(myNaN)); // true - the value is already NaN

console.log(isNaN(2 * myNumber)); // false - the multiplication result of two numbers is a number

console.log(isNaN(myNumber * myNaN)); // true - as the result of any maths involving NaN is NaN```

console.log() is used to output the return value of each example of isNaN().