The ?? operator: keeping zero instead of replacing it with a default

Useful to know

The operator ?? returns the right value only if the left is null or undefined. In other cases, it retains the left, including when it is 0, false or ''. This is important when the "no value" differs from the "value is zero". [1]

For example, const stock = 0; const shown = stock ?? 10; gives 0. If stock is undefined, the result is 10. The number 10 here is a notional value by default, not a real availability.

Do not automatically use a reserve value for each empty field. In a user form, empty text can mean "not filled in", but for ?? it is not null or undefined. This rule must be determined separately.

In a catalog, a free product with a price of 0 should not become a product with a default price. Distinguishing absence from a valid zero value prevents such silent errors in performance.