Необязательные параметры и параметры по умолчанию

function greet(name: string, greeting?: string): string {
  return (greeting ?? 'Hello') + ', ' + name;
}

function greet(name: string, greeting: string = 'Hello'): string {
  return greeting + ', ' + name;
}
greet('Alice');         // 'Hello, Alice'
greet('Bob', 'Hi');    // 'Hi, Bob'

function opt(x?: number) { return x; }     // x: number | undefined
function def(x: number = 0) { return x; }  // x: number

// Correct order
function create(name: string, role: string = 'user', active?: boolean) {}

// Error: required after optional
// function bad(x?: string, y: string) {}

function log(msg: string, level: string = 'info') { console.log(level, msg); }
log('test', undefined); // level is 'info'

function connect({ host = 'localhost', port = 5432 } = {}) {
  return `${host}:${port}`;
}

function joinAll(parts: string[], sep: string = ', '): string {
  return parts.join(sep);
}

interface Config { onReady?: () => void; }
function init(config: Config) {
  config.onReady?.();
}

function process(id?: number) {
  if (id === undefined) return;
  console.log(id * 2); // id is number here
}