Fraser Hamilton

Nullish Coalescing in Javascript

November 03, 2021

A Javascript operator that can be often overlooked is the Nullish Coalescing Operator represented as ??. In this article we will take a look at what it does and where it can be used.

It’s fairly common to come across javascript code in which the OR opeartor || is used to set a default value for a variable as can be seen below.

const myGreeting = greeting || 'Hello!'

Unfortunatley unbeknownst to some using the OR operator for this purpose can actually have unintended consequences as it will fall back to the default value whenever the variable evaluates to something falsy. Let’s take a look at an example in which we are setting the default value for a number.

const myNumber = number || 12

If number = 15 then the code above would evaluate such that myNumber = 15. If number = null then it will use the default resulting in myNumber = 12.

So far so good but what if we set number = 0 ? We’d run into trouble because although number has a value it’s a value that when evaluated as boolean is equivalent to false so we’d wind up falling back to our default number = 12.

This is where the Nullish Coalescing Operator can come in very handy by switching out || for ?? we can guarantee the default value will only be used when number = null or number = undefined.

const myNumber = number ?? 12


Written by Fraser Hamilton a full stack developer based out of Edinburgh, Scotland.

© 2021, Fraser Hamilton