Generic constraints

A type parameter with no constraint accepts any type, and that's exactly why it lets you do almost nothing with it. Inside findById<T>, item.id is a type error, because T could be a number or a string with no id.

A constraint spells out what the function accepts. function longest<T extends { length: number }>(a: T, b: T): T accepts strings, arrays and anything else with a length, rejects numbers, and lets you use a.length inside.

The common mistake is to drop the type parameter and write items: { id: number }[]. That accepts the books, but the result becomes just { id: number }, so book.title turns into an error. A constraint requires id and keeps the rest of the fields with it.