5. Generics

function first<T>(items: T[]): T | undefined { return items[0]; }
const n = first([1, 2, 3]);           // T inferred as number

interface ApiResult<T> { data: T; error?: string; }
interface Paged<T> { items: T[]; total: number; page: number; }

// Constraint: T must have an id
function byId<T extends { id: number }>(items: T[], id: number): T | undefined {
  return items.find(i => i.id === id);
}

// keyof constraint: safe property access
function pluck<T, K extends keyof T>(items: T[], key: K): T[K][] {
  return items.map(i => i[key]);
}
pluck(orders, "status");   // OrderStatus[] — "statuz" would be a compile error

// Generic class
class Cache<K, V> {
  private map = new Map<K, V>();
  get(key: K) { return this.map.get(key); }
  set(key: K, value: V) { this.map.set(key, value); }
}