Extending & Composing: Building Types From Other Types

Instead of repeating fields, TypeScript lets you build bigger types out of smaller ones. With interfaces, you use the extends keyword to inherit everything from a parent and add more:

interface Animal {
  name: string;
}
interface Dog extends Animal {
  breed: string;
}
// Dog now requires BOTH name and breed
const d: Dog = { name: "Rex", breed: "Lab" };

With type aliases, you compose using the intersection operator &, which merges members from every type into one. A & B means "has everything in A and everything in B":

type Timestamps = { createdAt: number };
type User = { id: string } & Timestamps;
// User requires id AND createdAt

The two approaches mix freely: an interface can extends a type alias, and a type can intersect an interface. A handy rule of thumb — extends is for interfaces and reads like inheritance, while & works on any types and reads like "add these together." Both let you reuse existing shapes rather than copy-pasting fields, so a change in the base flows to everything built on it.