TypeScript Academy · Lesson

Generic Constraints: Narrowing Type Parameters

Use extends to enforce shape requirements on generics.

Welcome

Advanced generic constraints let you narrow what types can be used as type arguments, enabling type-safe operations on complex shapes.

T extends U restricts T to types that are subtypes of U. This gives TypeScript knowledge about T's shape inside the function.

function len<T extends { length: number }>(val: T): number {
  return val.length;
}