JavaScript Best Practices

JavaScript Best Practices for Writing More Robust Code — Checking for Unexpected Values

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JavaScript is an easy to learn programming language. It’s easy to write programs that run and does something. However, it’s hard to account for all the uses cases and write robust JavaScript code.

In this article, we’ll look at ways to check for unexpected values, including null and NaN .

Checking for null

null is also a value that we may unexpectedly encounter, so we should also check for that.

Checking for null is a bit tricky since we can’t use the typeof operator to check for null as typeof null returns 'object' .

However, we can still use the === operator or the method to check for null .

We can use the === operator to check for null as follows:

if (foo === null) {  

In the code above, we just use the === operator to check for null as we would do for any other value.

The === operator doesn’t do any data type coercion, so it’ll compare the value as-is with any data type conversions before doing the comparison.

Therefore, the code above will work for null comparisons since it doesn’t try to convert null to anything else before doing the comparison. is a static object method that takes 2 arguments that take 2 objects that are to be compared.

It’ll return true is both arguments are null , so we can use that for checking if something is null .

To check for null with , we can write the following code:

if (, null)) {  

In the code above, we called with foo and null . Then if foo is null , will return true .

Check if a Value is NaN

NaN is a value that we’ll encounter if we try to convert something that’s not a number into a number or if we try to divide 0 or a non-number by 0.

There’re many ways to check for NaN , including the isNaN function, the Number.isNaN method, and .

The === and == operators won’t work because NaN is considered to be different from itself if we use these operators. Therefore NaN === NaN and NaN == NaN both return false .

However, there’re still many options to check for NaN .

The globalisNaN function can be used to check if a value is NaN or not. Data type coercion is done before checking for NaN with this function.

If we pass in something that will be NaN if we convert it to a number, then isNaN will return true . Otherwise, it’ll return false .

For instance, if we have:


then they all return true since they all return NaN if we try to convert them to a number.

On the other hand, if we have:


These will return true since true coerces to 1 and null coerces to 0 before being compared with isNaN to check if it’s NaN .

Also, the following returns true :


since parseInt(‘456ABC’); returns 456, but Number('456ABC') returns NaN .

The Number.isNaN function checks if a value is NaN but doesn’t try to convert it to a number before doing the comparison. This method is new to ES2015.

For instance, we can use that for comparing value as follows:

Number.isNaN('foo' / 0);  
Number.isNaN(0 / 0);

They all return true since all the expressions that we put into the argument are evaluated to NaN .

Anything that isn’t NaN will return false , so expressions like:


will return false in addition to numbers in the argument. It only checks if the value is NaN without trying to convert a number first.

Likewise, we can use the method to check if a value is NaN . Like Number.isNaN , it won’t try to do any data type coercion before doing the equality comparison with NaN . considers NaN to be equal to itself, so we can use it to check for NaN .

Therefore, we can use it as we do with isNaN . For instance, we can write:, NaN);`(Number.NaN, NaN);``('foo' / 0, NaN);  
``(0 / 0, NaN);`

will all return true . Anything that isn’t explicitly evaluated as NaN will returns false when comparing against NaN will . For instance:


returns false .


Checking for null and NaN have their own tricks. We can use === to check for null , in addition to .

To check for NaN , we can’t use === to check for NaN as NaN === NaN returns false . However, we can use , isNaN or Number.isNaN to do the comparison.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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