Fonctions génériques : paramètres de types

// Without generics — loses type info
function first(arr: any[]): any { return arr[0]; }

// With generics — type-safe
function first<T>(arr: T[]): T { return arr[0]; }

function identity<T>(value: T): T {
  return value;
}
const n = identity(42);       // T is number
const s = identity('hello');  // T is string

function wrap<T>(val: T): T[] { return [val]; }
const nums = wrap(42);    // T inferred as number -> number[]
const strs = wrap('hi');  // T inferred as string -> string[]

function pair<A, B>(a: A, b: B): [A, B] {
  return [a, b];
}
const p = pair(1, 'hello'); // [number, string]

function filterNull<T>(arr: (T | null)[]): T[] {
  return arr.filter((x): x is T => x !== null);
}

function create<T>(): T | null { return null; }
const user = create<User>(); // T cannot be inferred — must be explicit

function echo<T>(val: T): T { return val; }
const n: number = echo(42);   // OK
// const s: string = echo(42); // Error — T is number

const identity = <T>(val: T): T => val;
// In .tsx files:
const identity2 = <T,>(val: T): T => val;

function mapArray<T, U>(arr: T[], fn: (item: T) => U): U[] {
  return arr.map(fn);
}
const lengths = mapArray(['hello', 'world'], s => s.length);
// lengths: number[]

function createList<T = string>(): T[] { return []; }
const list = createList(); // T defaults to string