TypeScript

let n: number = 1;
let s: string = "x";
let ok: boolean = true;
let arr: number[] = [1, 2];
let tup: [string, number] = ["a", 1];
let any1: unknown;   // prefer over any
let n2: number | null = null;

interface User {
  id: number;
  name: string;
  role?: "admin" | "user";   // optional + union
  readonly created: Date;
}

type ID = string | number;
type Point = { x: number; y: number };

function add(a: number, b: number): number {
  return a + b;
}

const greet = (name: string = "world"): string =>
  `Hi ${name}`;

// overloads
function len(x: string): number;
function len(x: any[]): number;

function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

interface Box<T> { value: T }

// constraints
function prop<T, K extends keyof T>(o: T, k: K) {
  return o[k];
}

if (typeof x === "string") { /* string */ }
if (Array.isArray(x)) { /* array */ }
if ("id" in obj) { /* has id */ }
if (x instanceof Date) { /* Date */ }

// discriminated union
type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; s: number };