Объясните generics в TypeScript: что это, как работает и когда применять.

// Generic preserves the input→output relationship; inference fills T
function first<T>(arr: T[]): T | undefined {
  return arr[0];
}
const n = first([1, 2, 3]);      // T = number → number | undefined
const s = first(['a', 'b']);     // T = string → string | undefined

// ❌ any loses the type; caller gets no help downstream
function firstBad(arr: any[]): any { return arr[0]; }
const x = firstBad([1, 2]);      // x: any — unsafe from here on

// Multiple type params chain through composition
function map<T, U>(arr: T[], fn: (x: T) => U): U[] {
  return arr.map(fn);
}
map([1, 2], n => `#${n}`);       // T=number inferred, U=string inferred → string[]

// ❌ Unusable generic — T appears only in return; nothing to infer from
function create<T>(): T { return {} as T; } // caller MUST annotate; smell

// Generic container: remembers what it holds
class Stack<T> {
  private items: T[] = [];
  push(x: T) { this.items.push(x); }
  pop(): T | undefined { return this.items.pop(); }
}
const s2 = new Stack<string>(); // or inferred from first push in some cases