Интерфейс или тип: когда что использовать

interface A { x: number; }
type B = { x: number; };
// Both describe the same shape

interface Config { host: string; }
interface Config { port: number; } // OK — merged

type Settings = { host: string; };
// type Settings = { port: number; }; // Error

type ID = string | number;        // union — only with type
type Nullable<T> = T | null;       // union — only with type
type Point = [number, number];     // tuple — only with type

type ReadonlyUser = Readonly<{ name: string; age: number }>; // mapped type
type Keys = keyof User; // only with type

interface Repository<T> {
  findById(id: number): T | null;
  save(item: T): void;
}
class UserRepo implements Repository<User> { /* ... */ }

// interface
interface User { name: string; age: number; }

// type (union)
type ID = string | number;

// type (tuple)
type Coord = [number, number];