When we want to assign a default value to a variable, a common pattern is to use the logical OR opeartor (||):

Find out more about the JavaScript logical OR (||) operator in my blog post here.

let count = 0

let qty = count || 20

console.log(qty)
// 20

count when converted to a boolean is falsy, so count is falsy. So, the value of qty becomes 20.

This behavior can be problematic if we consider 0 as a valid value for qty.

We can avoid this pitfall by using the nullish coalescing operator, which returns the second operand when the first one evaluates to either null or undefined.

Syntax

leftExp ?? rightExp

The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

So, we can rewrite the above example using the nullish coalescing operator. This time, console.log(qty) returns 0.

let count = 0

let qty = count ?? 20

console.log(qty)
// 0