Nullish coalescing operator (??)
July 18, 2021
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.
This can be contrasted with the logical OR (||) operator, which returns the right-hand side operand if the left operand is any falsy value, not only null or undefined.
In other words, if you use || to provide some default value to another variable foo, you may encounter unexpected behaviors if you consider some falsy values as usable (e.g., ” or 0).
Examples
const nullValue = null;
const undefinedValue = undefined;
const emptyText = "";
const numberZero = 0;
nullValue || "default"; // "default"
undefinedValue || "default"; // "default"
emptyText || "default"; // "default"
numberZero || 99; // 99
nullValue ?? "default"; // "default"
undefinedValue ?? "default"; // "default"
emptyText ?? "default"; // "" (as the empty string is not null or undefined)
numberZero ?? 99; // 0