Lesson 16 — Generic constraints

Limit what T can be

A plain T can be anything, so TypeScript will not let you read t.length — a number has no length. A constraint with extends says "T can be any type, as long as it has these properties": <T extends { length: number }>. Strings and arrays pass; numbers do not.

function hasItems<T extends { length: number }>(x: T) {
  return x.length > 0;
}
hasItems("hi"); hasItems([1]); // ok
// hasItems(5); ← error

Why it matters: constraints let generic code use properties safely while staying flexible.