Default Values with the Nullish Coalescing Operator

Last reviewed on November 3, 2020

ES2020 brings the nullish coalescing operator to JavaScript, which allows us to create default values succinctly. The nullish coalescing operator is created with two question marks (??) and allows us to perform a logical OR only when the left value is null or undefined. For example:

const issueStatus = selectedStatus ?? 'open';

If selectedStatus is 'closed' (a truthy value), the issueStatus constant will be assigned 'closed'. If selectedStatus is undefined or null (which are falsey), the issueStatus constant will be assigned 'open'.

Now you might be wondering, why not use the logical OR operator (||)? For example:

const issueStatus = selectedStatus || 'open';

The difference here is that zero and an empty string are considered falsey, so if selectedStatus is either of those values, issueStatus will get assigned 'open'. With the nullish coalescing operator, the right side will only be evaluated if the left side is undefined or null. I’ve definitely been bitten by bugs where I intended for zero to be assigned as opposed to the default value in a logical OR expression.