Generic Interfaces and Type Aliases

Welcome

Generic Interface Syntax

interface Box<T> {
  value: T;
  unwrap(): T;
}
const box: Box<number> = { value: 42, unwrap() { return this.value; } };

Generic Type Alias

type Maybe<T> = T | null | undefined;
type Pair<A, B> = { first: A; second: B };
const p: Pair<string, number> = { first: 'Alice', second: 30 };

Generic Repository Interface

interface Repository<T> {
  findById(id: number): T | null;
  findAll(): T[];
  save(item: T): void;
  delete(id: number): void;
}

Implementing a Generic Interface

class UserRepo implements Repository<User> {
  findById(id: number): User | null { return null; }
  findAll(): User[] { return []; }
  save(user: User): void {}
  delete(id: number): void {}
}

Extending Generic Interfaces

interface Named<T> { name: T; }
interface NamedAndAged<T> extends Named<T> { age: number; }

Generic Response Type

type ApiResponse<T> = {
  status: 'success' | 'error';
  data: T;
  timestamp: number;
};
const res: ApiResponse<User[]> = { status: 'success', data: [], timestamp: Date.now() };

Multiple Type Parameters in Interfaces

interface KeyValue<K, V> {
  key: K;
  value: V;
}
const kv: KeyValue<string, number> = { key: 'age', value: 30 };

Generic Method in a Non-Generic Interface

interface Transformer {
  transform<T, U>(input: T, fn: (val: T) => U): U;
}

Recursive Generic Types

interface TreeNode<T> {
  value: T;
  children: TreeNode<T>[];
}

Type Alias vs Interface for Generics

Quick Check

Recap