// Declare variables
let x = 10;        // Block scoped (ES6)
const y = 5;       // Constant, also block scoped
var z = 3;         // Function scoped (legacy, avoid using)
 
// Data types
let name = "John";               // string
let age = 30;                    // number
let isOnline = true;             // boolean
let person = { name, age };      // object
let fruits = ["apple", "banana"]; // array
let nothing = null;              // null
let notDefined;                  // undefined

// Arithmetic
+, -, *, /, %, **
 
// Comparison
==, ===, !=, !==, >, <
 
// Logical
&&, ||, !
 
// Assignment
=, +=, -=, *=, /=
 
// Nullish & Optional
??   // Nullish Coalescing (if null or undefined)
?.   // Optional Chaining

if (condition) {
  // ...
} else if (anotherCondition) {
  // ...
} else {
  // ...
}
 
switch (value) {
  case 1:
    break;
  default:
    break;
}
 
for (let i = 0; i < 5; i++) { }
while (true) { }
do {
  // runs at least once
} while (false);
 
// Ternary
let result = condition ? "yes" : "no";

function greet(name) {
  return `Hello, ${name}`;
}
 
const greet = (name) => `Hello, ${name}`;
 
// Closure Example
function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}
 
const counter = outer();
counter(); // 1
counter(); // 2

const arr = [1, 2, 3, 4];
 
arr.push(5);         // ➕ Add to end
arr.pop();           // ➖ Remove from end
arr.unshift(0);      // ➕ Add to start
arr.shift();         // ➖ Remove from start
 
arr.includes(3);     // 🔍 Check if value exists
arr.indexOf(2);      // 🔍 Index of value
 
arr.slice(1, 3);     // 📦 Copy part of array
arr.splice(1, 1);    // 🔧 Remove/replace at index
 
// Iteration
arr.forEach(n => console.log(n));
 
// Transformation
const doubled = arr.map(n => n * 2);
 
// Filtering
const even = arr.filter(n => n % 2 === 0);
 
// Reducing
const sum = arr.reduce((a, b) => a + b, 0);
 
// Finding
const found = arr.find(n => n > 2);
 
arr.sort((a, b) => a - b);  // 🔃 Sort
arr.reverse();              // 🔁 Reverse

const str = "Hello, World!";
 
str.length;
str.toUpperCase();
str.toLowerCase();
str.includes("World");
str.indexOf("l");
str.lastIndexOf("l");
str.substring(0, 5);
str.slice(-6);
str.split(", ");
str.trim();
str.replace("World", "JS");
str.startsWith("Hello");
str.endsWith("!");

const user = {
  name: "Alice",
  age: 25,
  greet() {
    return `Hi ${this.name}`;
  }
};
 
user.name;
user["age"];
user.city = "NYC";
 
Object.keys(user);
Object.values(user);
Object.entries(user);
Object.hasOwn(user, 'age');
Object.assign({}, user);

const getData = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve("Done"), 1000);
  });
};
 
getData()
  .then(console.log)
  .catch(console.error);
 
// Async/Await
async function fetchData() {
  try {
    const res = await getData();
    console.log(res);
  } catch (e) {
    console.error(e);
  }
}

for (let item of arr) {
  console.log(item);
}
 
for (let index in arr) {
  console.log(arr[index]);
}
 
arr.forEach(item => console.log(item));

typeof 42;                // "number"
typeof null;              // "object" (quirk!)
Array.isArray([]);        // true
 
if (val == null) {
  // true for null or undefined
}
 
val !== null && typeof val === 'object';

// Selecting elements
const el = document.getElementById("id");
const all = document.querySelectorAll("div");
 
// Modifying elements
el.textContent = "Hello";
el.innerHTML = "<strong>Hi</strong>";
el.style.color = "red";
 
// Events
el.addEventListener("click", () => alert("Clicked!"));

console.log("Start");
setTimeout(() => console.log("Timeout"), 0);
Promise.resolve().then(() => console.log("Microtask"));
console.log("End");
 
// Output:
// Start
// End
// Microtask
// Timeout

localStorage.setItem("name", "John");
localStorage.getItem("name");
 
sessionStorage.setItem("id", "123");
 
document.cookie = "user=Ali; path=/; expires=Fri, 31 Dec 2025 23:59:59 GMT";

fetch("https://api.example.com")
  .then(res => res.json())
  .then(data => console.log(data));
 
async function fetchUser() {
  const res = await fetch("https://api.example.com/user");
  const user = await res.json();
  console.log(user);
}

const { name, age } = user; // Object Destructuring
const [a, b] = [1, 2];      // Array Destructuring
 
const add = (x, y = 0) => x + y;
 
const nums = [1, 2];
const more = [...nums, 3];
 
const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 };
 
// Optional chaining
user?.address?.city;
 
// Nullish coalescing
let val = name ?? "Guest";
 
// Object.hasOwn
Object.hasOwn(user, 'name');
 
// Template literals
const greetMsg = `Hello, ${name}`;

// export.js
export const name = "Imtiyaz";
export default function greet() { }
 
// import.js
import greet, { name } from "./export.js";

// Symbol
const sym = Symbol("id");
 
// Set
const set = new Set([1, 2, 2]);
set.add(3);
set.has(2);
set.delete(1);
 
// Map
const map = new Map();
map.set("name", "Ali");
map.get("name");
map.has("name");