Generic interfaces and generic classes in TypeScript

Generic interfaces

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

Any class implementing this interface must specify T:

class UserRepo implements Repository {
  private users: User[] = [];
  findById(id: number) { return this.users.find(u => u.id === id) ?? null; }
  findAll()            { return this.users; }
  save(user: User)     { this.users.push(user); }
  delete(id: number)   { this.users = this.users.filter(u => u.id !== id); }
}

Generic class: Stack

class Stack {
  private items: T[] = [];
  push(item: T): void   { this.items.push(item); }
  pop(): T | undefined  { return this.items.pop(); }
  peek(): T | undefined { return this.items[this.items.length - 1]; }
  get size(): number    { return this.items.length; }
}

const stack = new Stack();
stack.push(1);
stack.push(2);
stack.pop(); // 2