→ JavaScript Tutorial → JavaScript Number Object →

JavaScript isNaN() Function

This tutorial explores JavaScript's isNaN() function. Related to the Number object's NaN property (which stands for "Not a Number"), the isNaN() function enables you to check whether a value is a number or not.

Note that the isNaN() function only checks for the "numerical nature" of a value, not whether the value it is finite. To check if a number is finite, you should instead use JavaScript's isFinite() funtion.

Using JavaScript's isNaN() Function

Short of full-fledged try-catch error handling, JavaScript's isNaN() function is a good technique to check the validity of numerical values before using them. This is especially advisable when dealing with user input.

The isNaN() funtion returns true if the value passed was not a number, or false if the value passed was in fact a number. (Remember that "NaN" stands for "Not a Number".) In other words, the isNaN() function will not return an error in case of unexpected argument type, allowing you to elegantly handle the problem.

var attendees = "Four members present";

// Ensure that the value is numerical
if( isNaN(attendees) )
    alert("Please enter a number for the number of attendees.");

As expected, our script displayed the error message since the variable tested did not contain a numerical value. JavaScript isNaN() function

Using the isNaN() Function with Implicit Numerical Values

The isNaN() function is a bit more insightful than first meets the eye: it can determine if a value could be numerical with a proper conversion. A string-quoted number 3 does not yield the expected result.

// Pass a quoted number (string) as argument
alert( isNaN("3.14") );

The line of JavaScript code above produces the following: JavaScript isNaN() function with implicit numbers

In other words, the isNaN() function realizes that the quoted string is a number, which can straight-forwardly be converted to a number. As a result, it returned false (i.e. the string passed was in fact a number.)

The isNaN() function does consider infinity and numbers larger than MAX_VALUE as numbers, even if the JavaScript interpreter cannot handle them. In other words, isNaN() reliable tells you about the numerical nature of a value, whether it is finite or infinite.

alert( isNaN(Infinity) ); returns false (meaning that Infinity is numerical).

Don't blindly trust the isNaN() function

In some cases, you will have to deal with free form text entries from a form, which means that you may not have full control over the format in which numbers will be typed into the form. Using only the NaN property or its sibling, the isNaN() function, will accidentally condemn potentially valid inputs:

// Someone filling a form mistakenly typed a space after a number:
var numex = "2 ";

alert( isNaN(numex) ); // JavaScript gets it, and returns false

// This time, an unexpanded fraction:
numex = "1/2";

alert( isNaN(numex) ); // JS interpreter fails and returns true

In the script above, JavaScript fails to recognize a number through the isNaN() function, even if the value is clearly valid for humans. Just a little something to keep in mind for your validation code.

Clean up arguments passed to isNaN() with regular expressions

Here is a quick way to remove any non-digit characters from a string input (like a form field) before passing it to the isNaN() method:

// Here is an inprecise value typed in a form
var numex = "2 I think";

alert( isNaN(numex) ); // returns true (it isn't a number)

// Let's remove any non-digit characters:
numex = numex.replace(/\D/g,""); // numex value is now "2"

alert( isNaN(numex) ); // This time returns false (it is a number)

In the script above, we used JavaScript's replace() method with a regular expression to remove all non-digit characters from the input string. The regular expression literal "/\D/g" means "all the non-numbers" - in other words, remove all non-digits by replacing them with an empty string ("").

If you wanted to accept floating point values, you would have to allow for both digits and the period character; for more details on the topic, please see our JavaScript regular expressions tutorial.

Test the isNaN() Function

Interactively test the isNaN() function by editing the JavaScript code below and clicking the Test isNaN() function button.

Browser support for JavaScript isNaN() function