const name = "John";     // Constant
let age = 25;           // Mutable
var count = 10;         // Function scoped

const str = "Hello";          // String
const num = 42;               // Number
const bool = true;            // Boolean
const nothing = null;         // Null
const notDefined = undefined; // Undefined
const sym = Symbol('id');     // Symbol

// Arithmetic
console.log(10 + 5);    // 15
console.log(10 ** 2);   // 100

// Comparison  
console.log(5 > 3);     // true
console.log(5 === '5'); // false

// Logical
console.log(true && false); // false
console.log(true || false); // true

// If/else
if (age >= 18) {
  console.log("Adult");
} else {
  console.log("Minor");
}

// Ternary
const status = age >= 18 ? "Adult" : "Minor";

// Switch
switch (day) {
  case 'Monday': 
    console.log("Week start");
    break;
  default:
    console.log("Other day");
}

// For loop
for (let i = 0; i < 5; i++) {
  console.log(i);
}

// While loop
let count = 0;
while (count < 3) {
  count++;
}

// For...of
for (const item of array) {
  console.log(item);
}

// Function declaration
function greet(name) {
  return `Hello ${name}`;
}

// Arrow function
const add = (a, b) => a + b;

// Default parameters
function createUser(name, age = 18) {
  return { name, age };
}

const nums = [1, 2, 3];

// Common methods
nums.push(4);           // Add to end
nums.pop();             // Remove from end  
nums.map(n => n * 2);   // [2, 4, 6]
nums.filter(n => n > 1); // [2, 3]
nums.find(n => n > 1);  // 2

const person = {
  name: "Alice",
  age: 30,
  greet() {
    return `Hello ${this.name}`;
  }
};

// Access
person.name;            // "Alice"
person['age'];          // 30

// Destructuring
const { name, age } = person;

class Person {
  constructor(name) {
    this.name = name;
  }
  
  greet() {
    return `Hello ${this.name}`;
  }
}

class Student extends Person {
  constructor(name, grade) {
    super(name);
    this.grade = grade;
  }
}

// Create promise
const promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Done!"), 1000);
});

// Use promise
promise.then(result => console.log(result));

async function fetchData() {
  try {
    const response = await fetch('/api/data');
    const data = await response.json();
    return data;
  } catch (error) {
    console.error('Error:', error);
  }
}

// Array destructuring
const [first, second] = [1, 2, 3];

// Object destructuring  
const { name, age } = { name: "John", age: 30 };

// With defaults
const { theme = "light" } = options;

// Spread
const arr1 = [1, 2];
const arr2 = [...arr1, 3, 4]; // [1, 2, 3, 4]

const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 }; // {a:1, b:2}

// Rest
const [first, ...rest] = [1, 2, 3]; // rest = [2, 3]

const name = "Alice";
const age = 30;

// Basic template
const greeting = `Hello ${name}!`;

// Multi-line
const message = `
  Name: ${name}
  Age: ${age}
`;

// Export
export const PI = 3.14;
export function add(a, b) { return a + b; }
export default class Calculator {}

// Import
import Calculator, { PI, add } from './math.js';

const numbers = [1, 2, 3, 4];

numbers.map(x => x * 2);     // [2, 4, 6, 8]
numbers.filter(x => x > 2);  // [3, 4]
numbers.reduce((a, b) => a + b, 0); // 10
numbers.find(x => x > 2);    // 3
numbers.some(x => x > 3);    // true
numbers.every(x => x > 0);   // true

const person = { name: "John", age: 30 };

Object.keys(person);    // ['name', 'age']
Object.values(person);  // ['John', 30]
Object.entries(person); // [['name','John'], ['age',30]]

// Copy object
const copy = { ...person };
const assigned = Object.assign({}, person);

try {
  // Risky operation
  const result = riskyFunction();
} catch (error) {
  console.error('Caught error:', error.message);
} finally {
  console.log('Always runs');
}

// Set - unique values
const unique = new Set([1, 2, 2, 3]); // {1, 2, 3}

// Map - key-value pairs
const map = new Map();
map.set('name', 'Alice');
map.get('name'); // 'Alice'

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

// Safe access
const userName = user?.profile?.name; // "John"
const missing = user?.missing?.prop;  // undefined

// With nullish coalescing
const name = user?.name ?? 'Anonymous';

fetch('/api/users')
  .then(response => response.json())
  .then(users => console.log(users))
  .catch(error => console.error(error));

// Store data
localStorage.setItem('user', 'Alice');

// Retrieve data
const user = localStorage.getItem('user');

// Remove data
localStorage.removeItem('user');

// Run once after delay
setTimeout(() => {
  console.log('After 2 seconds');
}, 2000);

// Run repeatedly
const interval = setInterval(() => {
  console.log('Every second');
}, 1000);

// Clear interval
clearInterval(interval);