JS ES6+ Features

// Array destructuring
const [first, second, ...rest] = [1, 2, 3, 4, 5];
// first=1, second=2, rest=[3,4,5]

// Object destructuring
const { name, age = 18 } = { name: "Alice" };
// name="Alice", age=18 (default)

// Function parameter destructuring
function display({ name, age }) {
  console.log(`${name} is ${age}`);
}
display({ name: "Alice", age: 25 });

// Spread — expand array/object
const a = [1, 2, 3];
const b = [...a, 4, 5];          // [1, 2, 3, 4, 5]
const obj = { ...user, role: "admin" };

// Rest — collect remaining args
function sum(...nums) {
  return nums.reduce((t, n) => t + n, 0);
}
sum(1, 2, 3, 4);  // 10

// math.js — export
export function add(a, b) { return a + b; }
export const PI = 3.14159;
export default class Calculator { /* ... */ }

// app.js — import
import Calculator, { add, PI } from "./math.js";
import * as Math from "./math.js";

// Promise
fetch("/api/users")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

// async/await — cleaner version
async function loadUsers() {
  try {
    const res  = await fetch("/api/users");
    const data = await res.json();
    console.log(data);
  } catch (err) {
    console.error("Failed:", err);
  }
}

loadUsers();

const user = { profile: { name: "Alice" } };

// Optional chaining — safe deep access
const city = user?.profile?.city;  // undefined (no error)
const len  = user?.friends?.length ?? 0;  // 0

// Nullish coalescing — fallback only for null/undefined (not 0 or "")
const port = config.port ?? 3000;
const name = input ?? "Guest";