What is NaN in JavaScript?

In the world of JavaScript programming, there’s a particular value that often causes confusion and errors – NaN, which stands for Not-A-Number. Despite its name, NaN is a numeric data type used to represent a value that is undefined or not a number in conventional terms. In this blog post, we’ll explore what NaN is, how to check for it, and its implications for JavaScript developers. Understanding NaN in JavaScript is crucial for writing robust and error-free code.

Key Takeaways

  • NaN is a special value in JavaScript representing a computational anomaly or an undefined result in numerical operations.
  • There are built-in functions and operations to test whether a value is NaN, such as Number.isNaN() and isNaN().
  • Understanding how NaN behaves and propagates in expressions can help prevent bugs and unexpected behaviors in your JavaScript programs.
  • JavaScript developers at any level should be familiar with NaN to effectively debug and write accurate numerical computations.

What is NaN in JavaScript?

NaN (Not-A-Number) is a property in JavaScript used to indicate a value that is not a valid number. It is a unique type of value that results from an operation that doesn’t yield a meaningful numeric result.

A Step-by-Step Guide to NaN in JavaScript

Here is a simplified guide to understanding NaN in JavaScript:

  1. Recognize that NaN is produced by operations that do not result in a clear numeric answer.
  2. Familiarize yourself with Number.isNaN() and isNaN(), methods to check for NaN.
  3. Remember, NaN is not equal to anything, including itself.
  4. Be cautious of cases where NaN might propagate through your calculations without any error being raised.

What Causes NaN in JavaScript?

NaN is encountered in JavaScript under specific circumstances. Here are common scenarios where NaN can arise:

  • Math operations that are undefined, such as dividing 0 by 0.
  • Conversions of non-numeric strings or symbols to numbers.
  • Failures of mathematical functions, like taking the square root of a negative number.

Understanding that NaN appears in specific situations allows developers to anticipate its occurrence and handle it gracefully in their code.

Testing Against NaN

How do you determine if a value is actually NaN? Unlike other types of checks, simply using equality checks (like ===) with NaN is ineffective because NaN is the only JavaScript value that is not equal to itself. Instead, JavaScript provides specific functions to test for NaN values:

  • Number.isNaN(): This function determines if the value is NaN and doesn’t coerce the argument to a number. It’s the recommended method for modern code.
  • isNaN(): It also checks for NaN, but it first converts the argument to a number, which sometimes yields unexpected results and is generally less reliable.

Using these functions is crucial to correctly identify NaN values in your code.

The Unique Behavior of NaN

An intriguing trait of NaN is that it is not equal to any other value, including itself. This means NaN === NaN or NaN == NaN will both return false. This characteristic is unlike any other in JavaScript and has significant implications for the equality comparison of numeric values.

Another noteworthy behavior is the propagation of NaN: if any part of a mathematical expression results in NaN, the whole expression will typically also result in NaN. This “infectious” nature of NaN can be useful for indicating an issue at any point in a chain of computations.

Browser Compatibility

Developers should note the function they use to check for NaN and its browser compatibility, as older browsers may not support Number.isNaN(). Be sure to check the compatibility tables on resources like MDN for the methods you intend to use.


Understanding NaN in JavaScript is essential for anyone working with numerical computations. It can pose challenges but knowing when it appears and how to check for it can make it manageable. Use the provided functions to verify NaN and always be mindful of its strange behavior in comparisons and calculations. Remember, whenever you’re in doubt, MDN Web Docs is a reliable resource to refer to for any JavaScript-related questions.

Call to Action

If you’re dealing with numbers in JavaScript, take the time to truly understand NaN and its implications. Experiment with the examples provided in this article to see NaN in action. And, always keep learning to stay ahead in the fast-evolving world of JavaScript!