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You'll learn about JavasScript NaN in this tutorial, how to check whether a value is NaN, and how to effectively handle NaN.

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What is NaN in Javascript?

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NaN, in JavaScript, can be many things. In fact, it can be almost anything, so long as it is Not a Number. Its type is technically “number” (when evaluated with “typeof”), although it stands for Not a Number.

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Values in NaN Javascript

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Values can become NaN through a variety of means, which usually involve erroneous math calculations (such as 0/0), or as a result of type coercion, either implicit or explicit.

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A common example is when you run parseInt on a string that starts with an alphabetical character. This isn’t exclusive to parseInt, as it also applies when using explicit coercion with Number(), or with the unary “+” operator.

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How to check NaN in Javascript?

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Before selecting a method for checking for NaN, how should you not check for NaN?

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NaN is a bizarre value in JavaScript, as it does not equal itself when compared, either with the loose equality (==) or strict equality (===) operator. NaN is the only value in the entire language which behaves in this manner with regards to comparisons.

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For example, if parseInt(“a”) returns NaN, then parseInt(“a”) === NaN will return false. This may seem strange, but it makes perfect sense after thinking about what NaN really is.

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NaN doesn’t tell you what something is, it tells you what it isn’t.

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These two different strings being passed to parseInt() will both return NaN.

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parseInt(“abc”) // NaN\nparseInt(“def”) // NaN
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Both statements return NaN, but are they really the same? Maybe, but it certainly makes sense why JavaScript would disagree, given that they are derived from different string arguments.

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Here are a few examples of strict inequality comparisons, which demonstrate the inconsistency of NaN.

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2 !== 2 // false\ntrue !== true // false\n“abc” !== “abc” // false\n...\nNaN !== NaN // true
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Method 1: isNaN or Number.isNaN

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JavaScript has a built-in method, appropriately named “isNaN,” which checks for NaN. There is a newer function called Number.isNaN, which is included in the ES2015 spec.

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The difference between isNaN and Number.isNaN is that isNaN coerces the argument into a number type. To avoid complicated and unexpected outcomes, it is often advised to use the newer, more robust Number.isNaN to avoid these side effects.

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Number.isNaN does not perform any forcible type conversion, so it simply returns the boolean based on the parameter.

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Here is an example of the difference between the two methods:

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isNaN(undefined) // true\nNumber.isNaN(undefined) // false
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isNaN, when passed undefined, returns true because undefined becomes NaN after number coercion. You can test this yourself by running Number(undefined). You will find that it returns NaN.

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Number.isNaN, on the other hand, returns false. This is because no coercion takes place, and undefined is not NaN, it is simply undefined.

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It is also important to note that Number.isNaN is a newer (ES2015) method in JavaScript, so browser support for Number.isNaN is not as stable as isNaN, which has been around since ES1 (1997).

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Method 2: Object.is

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Object.is is a JavaScript method which checks for sameness. It generally performs the same evaluations as a strict equality operator (===), although it treats NaN differently from strict equality.

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Object.is(0, -0) will return false, while 0 === -0 will return true. Comparisons of 0 and -0 differ, as do comparisons of NaN. This concept is called “Same-value-zero equality.”

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NaN === NaN // false\nObject.is(NaN, NaN) // true
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Object.is(NaN, NaN) will in fact return true, while we already know that NaN === NaN returns false. That makes this yet another valid way to check if something is not a number.

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Conclusion

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Between the given methods of checking for NaN, the most common is to use the global isNaN function, or the ES2015 Number.isNaN method. While method #2 is valid, most people will typically use isNaN or Number.isNaN, which were created specifically for checking for NaN.

\n","headings":[{"value":"What is NaN in Javascript?","depth":2},{"value":"Values in NaN Javascript","depth":2},{"value":"How to check NaN in Javascript?","depth":2},{"value":"Method 1: isNaN or Number.isNaN","depth":3},{"value":"Method 2: Object.is","depth":3},{"value":"Conclusion","depth":2}],"fields":{"slug":"/engineering/checking-for-nan-in-javascript/"},"frontmatter":{"metatitle":null,"metadescription":null,"description":"In this guide you'll learn about JavasScript NaN, how to verify whether a value is NaN, and how to manage NaN effectively.","title":"NaN in JavaScript: An Essential Guide","canonical":null,"date":"November 22, 2019","updated_date":null,"tags":["JavaScript"],"coverImage":{"childImageSharp":{"fluid":{"aspectRatio":1.3333333333333333,"src":"/static/64b659badd7a09c3d955b9c49a7f271f/f1752/numbers-1.webp","srcSet":"/static/64b659badd7a09c3d955b9c49a7f271f/1c9b5/numbers-1.webp 200w,\n/static/64b659badd7a09c3d955b9c49a7f271f/f1752/numbers-1.webp 400w","sizes":"(max-width: 400px) 100vw, 400px"}}},"author":{"id":"Greg Sakai","github":null,"bio":"Greg Sakai is a Software Developer at LoginRadius. He is currently studying Computer Science part time, and he loves all things JavaScript. In his spare time, he likes doing homework.","avatar":null}}}},"pageContext":{"id":"0f63d421-ce01-5020-8b12-991083fd5274","fields__slug":"/engineering/checking-for-nan-in-javascript/","__params":{"fields__slug":"engineering"}}},"staticQueryHashes":["1171199041","1384082988","1711371485","1753898100","2100481360","229320306","23180105","528864852"]}