// let: block-scoped, mutable
let count = 0;
count = 1;

// const: block-scoped, immutable reference (objects/arrays can still be mutated)
const user = { name: "Alice" };
user.name = "Bob"; // Allowed

// var: function-scoped, hoisted (avoid using in modern JS)
var oldWay = true;

// Function Declaration (hoisted)
function greet(name) {
  return `Hello, ${name}!`;
}

// Function Expression
const greetExpr = function(name) {
  return `Hello, ${name}!`;
};

// Arrow Function (lexical `this`)
const add = (a, b) => a + b;
const multiline = (x) => {
  const result = x * 2;
  return result;
};

// Default Parameters
const multiply = (a, b = 1) => a * b;

const arr = [1, 2, 3, 4, 5];

// Mutating methods
arr.push(6);      // Add to end
arr.pop();        // Remove from end
arr.unshift(0);   // Add to start
arr.shift();      // Remove from start
arr.splice(1, 2); // Remove/replace items at index

// Non-mutating (pure) methods
const doubled = arr.map(x => x * 2);
const evens = arr.filter(x => x % 2 === 0);
const sum = arr.reduce((acc, curr) => acc + curr, 0);
const found = arr.find(x => x > 3);
const hasThree = arr.includes(3);

// Destructuring & Spread
const [first, second, ...rest] = arr;
const combined = [...arr, 6, 7, 8];

const person = {
  name: "Jane",
  age: 28,
  greet() {
    console.log("Hi!");
  }
};

// Accessing properties
person.name;
person["age"];

// Object Methods
Object.keys(person);    // ['name', 'age', 'greet']
Object.values(person);  // ['Jane', 28, f]
Object.entries(person); // [['name', 'Jane'], ...]

// Destructuring & Rest/Spread
const { name, age: userAge } = person;
const clone = { ...person, location: "NY" };

// Optional Chaining & Nullish Coalescing
const zip = person.address?.zipCode ?? "00000";

// Creating a Promise
const fetchData = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve("Data loaded!"), 1000);
  });
};

// Using .then().catch()
fetchData()
  .then(data => console.log(data))
  .catch(err => console.error(err));

// Async/Await (Modern approach)
async function getData() {
  try {
    const data = await fetchData();
    console.log(data);
  } catch (error) {
    console.error("Failed:", error);
  }
}

// Promise Combinators
Promise.all([p1, p2]);     // Waits for all to resolve, or any to reject
Promise.allSettled([...]); // Waits for all, regardless of reject/resolve
Promise.race([...]);       // Returns first to finish (resolve or reject)
Promise.any([...]);        // Returns first to resolve

const name = "World";
const greeting = `Hello, ${name}!
You can even have
multi-line strings.`;

// Named Export
export const API_KEY = "123";
export function helper() {}

// Default Export
export default class User {}

// Importing
import User, { API_KEY, helper } from './module.js';
import * as utils from './utils.js';