Comparison with NaN in Javascript | isNaN() Function JS

Node.js / September 22, 2020 / TopCode / 0

Introduction

In this blog post, You will learn about a common JavaScript mistake which most of the developers don’t know.

Let’s assume you are doing some string manipulation and it resulted into NaN (Not-a-Number). Suppose you are using Javascript’s parseInt() function as follows:

let x = parseInt("Foo")

As we know that “Foo” is not a integer value, therefore variable x will result into NaN. Which means “x” is Not-a-Number type.

Now suppose you want to check whether variable x is NaN or not. Firstly,You will try following cases:

if(x===NaN)  //incorrect
if(x==="NaN") //incorrect

Arithmetic comparisons with the not-a-number value NaN are useless: nothing is considered to be equal to NaN, not even NaN itself, and similarly nothing is considered greater or less than NaN

Recommandation

Use the isNaN() function from the standard library to determine whether a given value is NaN. isNaN() function determines whether a value is an illegal number (Not-a-Number). This isNaN() function returns true if the value is equal to NaN. Otherwise, it returns false.

Always use isNaN() function to compare NaN value as follows:

if(isNaN(x)) // correct way to check NaN

Following are some examples of NaN:

isNaN('NaN') //true
isNaN(1) //false
isNaN(NaN) //true
isNaN('') //false
isNaN(15-2) //false
isNaN(undefined) //true
isNaN('HelloWorld') //true
isNaN(0 / 0) //true
isNaN(null) //false
isNaN('345') //false
isNaN(-41.53) //false
isNaN('2020/10/10') //true
isNaN(true) //false

References

• Mozilla Developer Network: NaN.

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