function varExample() {
  var x = 1;
  if (true) {
    var x = 2; // Same variable!
    console.log(x); // 2
  }
  console.log(x); // 2
}

function letExample() {
  let x = 1;
  if (true) {
    let x = 2; // Different variable
    console.log(x); // 2
  }
  console.log(x); // 1
}

const PI = 3.14159;
// PI = 3; // Error: Assignment to constant variable

const user = { name: 'John' };
user.name = 'Jane'; // OK - modifying object properties
// user = {}; // Error - reassigning reference

// Type checking
typeof "hello"        // "string"
typeof 42             // "number"
typeof true           // "boolean"
typeof undefined      // "undefined"
typeof null           // "object" (historical bug!)
typeof {}             // "object"
typeof []             // "object"
typeof function(){}   // "function"

// Better array checking
Array.isArray([])     // true
Array.isArray({})     // false

// Null checking
value === null        // true only for null
value == null         // true for null AND undefined

function greet(name) {
  return `Hello, ${name}!`;
}

const greet = function(name) {
  return `Hello, ${name}!`;
};

const greet = (name) => `Hello, ${name}!`;

// Multiple parameters
const add = (a, b) => a + b;

// Single parameter (parentheses optional)
const double = x => x * 2;

// Multiple statements (need braces and explicit return)
const complexFunction = (x, y) => {
  const result = x + y;
  return result * 2;
};

// Object literal
const person = {
  name: 'John',
  age: 30,
  greet() {
    return `Hello, I'm ${this.name}`;
  }
};

// Accessing properties
person.name           // Dot notation
person['name']        // Bracket notation (dynamic keys)

// Adding/modifying properties
person.email = 'john@example.com';
person.age = 31;

// Deleting properties
delete person.email;

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

// Transformation
numbers.map(n => n * 2)           // [2, 4, 6, 8, 10]
numbers.filter(n => n > 2)        // [3, 4, 5]
numbers.reduce((sum, n) => sum + n, 0) // 15

// Iteration
numbers.forEach(n => console.log(n));

// Search
numbers.find(n => n > 3)          // 4
numbers.findIndex(n => n > 3)     // 3
numbers.includes(3)               // true

// Mutation (modify original)
numbers.push(6)                   // Add to end
numbers.pop()                     // Remove from end
numbers.unshift(0)                // Add to start
numbers.shift()                   // Remove from start
numbers.splice(2, 1)              // Remove 1 item at index 2

// Non-mutating
numbers.slice(1, 3)               // [2, 3]
numbers.concat([6, 7])            // [1, 2, 3, 4, 5, 6, 7]
[...numbers, 6, 7]                // Spread operator

function createCounter() {
  let count = 0; // Private variable

  return {
    increment() {
      return ++count;
    },
    decrement() {
      return --count;
    },
    getCount() {
      return count;
    }
  };
}

const counter = createCounter();
console.log(counter.increment()); // 1
console.log(counter.increment()); // 2
console.log(counter.getCount());  // 2
// count is not directly accessible

// Constructor function
function Animal(name) {
  this.name = name;
}

// Add method to prototype
Animal.prototype.speak = function() {
  return `${this.name} makes a sound`;
};

const dog = new Animal('Dog');
console.log(dog.speak()); // "Dog makes a sound"

// Inheritance
function Dog(name, breed) {
  Animal.call(this, name); // Call parent constructor
  this.breed = breed;
}

// Set up prototype chain
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

Dog.prototype.bark = function() {
  return `${this.name} barks!`;
};

const myDog = new Dog('Buddy', 'Golden Retriever');
console.log(myDog.speak()); // "Buddy makes a sound" (inherited)
console.log(myDog.bark());  // "Buddy barks!" (own method)

class Animal {
  constructor(name) {
    this.name = name;
  }

  speak() {
    return `${this.name} makes a sound`;
  }

  // Static method
  static create(name) {
    return new Animal(name);
  }
}

class Dog extends Animal {
  constructor(name, breed) {
    super(name); // Call parent constructor
    this.breed = breed;
  }

  speak() {
    return `${super.speak()} - Woof!`;
  }

  // Getter
  get info() {
    return `${this.name} is a ${this.breed}`;
  }

  // Setter
  set nickname(value) {
    this._nickname = value;
  }
}

const myDog = new Dog('Buddy', 'Golden Retriever');
console.log(myDog.speak());     // "Buddy makes a sound - Woof!"
console.log(myDog.info);        // "Buddy is a Golden Retriever"
myDog.nickname = 'Bud';

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

// Object destructuring
const user = { name: 'John', age: 30, city: 'NYC' };
const { name, age } = user;
console.log(name); // "John"

// Renaming
const { name: userName, age: userAge } = user;

// Default values
const { country = 'USA' } = user;

// Nested destructuring
const data = {
  user: { name: 'John', address: { city: 'NYC' } }
};
const { user: { address: { city } } } = data;

// Function parameters
function greet({ name, age = 0 }) {
  return `${name} is ${age} years old`;
}
greet({ name: 'John', age: 30 }); // "John is 30 years old"

// Arrays
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5]; // [1, 2, 3, 4, 5]
const combined = [...arr1, ...arr2];

// Objects
const obj1 = { a: 1, b: 2 };
const obj2 = { ...obj1, c: 3 }; // { a: 1, b: 2, c: 3 }

// Function arguments
Math.max(...arr1); // 3

// Shallow copy
const copy = [...arr1];
const objCopy = { ...obj1 };

// Function parameters
function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}
sum(1, 2, 3, 4); // 10

// With other parameters
function multiply(multiplier, ...numbers) {
  return numbers.map(n => n * multiplier);
}
multiply(2, 1, 2, 3); // [2, 4, 6]

const name = 'John';
const age = 30;

// String interpolation
const greeting = `Hello, ${name}!`;

// Expressions
const message = `In 5 years, you'll be ${age + 5}`;

// Multi-line
const multiline = `
  This is a
  multi-line
  string
`;

// Tagged templates
function highlight(strings, ...values) {
  return strings.reduce((result, str, i) => {
    return result + str + (values[i] ? `<mark>${values[i]}</mark>` : '');
  }, '');
}

const html = highlight`Hello, ${name}! You are ${age} years old.`;
// "Hello, <mark>John</mark>! You are <mark>30</mark> years old."

const user = {
  name: 'John',
  address: {
    city: 'NYC'
  }
};

// Without optional chaining
const city = user && user.address && user.address.city;

// With optional chaining
const city = user?.address?.city; // "NYC"
const zip = user?.address?.zip;   // undefined (no error!)

// With methods
user.getName?.(); // Only calls if method exists

// With arrays
const firstItem = arr?.[0];

// || returns right side for ANY falsy value
const value1 = 0 || 'default';     // "default"
const value2 = '' || 'default';    // "default"
const value3 = false || 'default'; // "default"

// ?? only for null/undefined
const value4 = 0 ?? 'default';     // 0
const value5 = '' ?? 'default';    // ""
const value6 = false ?? 'default'; // false
const value7 = null ?? 'default';  // "default"
const value8 = undefined ?? 'default'; // "default"

// Named exports
export const PI = 3.14159;
export function add(a, b) {
  return a + b;
}
export class Calculator {
  // ...
}

// Export multiple
const multiply = (a, b) => a * b;
const divide = (a, b) => a / b;
export { multiply, divide };

// Default export (one per file)
export default class App {
  // ...
}

// Or
class App { }
export default App;

// Named imports
import { PI, add } from './math.js';
import { multiply as mult } from './math.js'; // Rename

// Default import
import App from './App.js';

// Mix default and named
import App, { PI, add } from './App.js';

// Import all
import * as Math from './math.js';
Math.PI;
Math.add(1, 2);

// Import for side effects only
import './polyfills.js';

console.log('1');

setTimeout(() => {
  console.log('2');
}, 0);

Promise.resolve().then(() => {
  console.log('3');
});

console.log('4');

// Output: 1, 4, 3, 2
// Microtasks (Promises) have priority over macrotasks (setTimeout)

function fetchUser(userId, callback) {
  setTimeout(() => {
    callback(null, { id: userId, name: 'John' });
  }, 1000);
}

function fetchPosts(userId, callback) {
  setTimeout(() => {
    callback(null, [{ id: 1, title: 'Post 1' }]);
  }, 1000);
}

// Callback hell
fetchUser(1, (error, user) => {
  if (error) {
    console.error(error);
    return;
  }

  fetchPosts(user.id, (error, posts) => {
    if (error) {
      console.error(error);
      return;
    }

    console.log(user, posts);
  });
});

// Creating a promise
const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = true;
    if (success) {
      resolve('Success!');
    } else {
      reject(new Error('Failed!'));
    }
  }, 1000);
});

// Using a promise
promise
  .then(result => {
    console.log(result); // "Success!"
    return result.toUpperCase();
  })
  .then(upperResult => {
    console.log(upperResult); // "SUCCESS!"
  })
  .catch(error => {
    console.error(error);
  })
  .finally(() => {
    console.log('Cleanup');
  });

// Promise utilities
Promise.all([promise1, promise2, promise3])
  .then(results => {
    // All resolved: [result1, result2, result3]
  });

Promise.race([promise1, promise2])
  .then(result => {
    // First to resolve
  });

Promise.allSettled([promise1, promise2])
  .then(results => {
    // All completed (resolved or rejected)
    // [{ status: 'fulfilled', value: ... }, { status: 'rejected', reason: ... }]
  });

Promise.any([promise1, promise2])
  .then(result => {
    // First to fulfill (resolve)
  });

async function fetchUserData(userId) {
  try {
    const user = await fetchUser(userId);
    const posts = await fetchPosts(user.id);
    const comments = await fetchComments(posts[0].id);

    return { user, posts, comments };
  } catch (error) {
    console.error('Error:', error);
    throw error;
  }
}

// Using the async function
fetchUserData(1)
  .then(data => console.log(data))
  .catch(error => console.error(error));

// Parallel execution
async function fetchAllData() {
  const [users, posts, comments] = await Promise.all([
    fetchUsers(),
    fetchPosts(),
    fetchComments()
  ]);

  return { users, posts, comments };
}

// Sequential vs Parallel
async function sequential() {
  const result1 = await operation1(); // Wait
  const result2 = await operation2(); // Then wait
  return [result1, result2];
}

async function parallel() {
  const [result1, result2] = await Promise.all([
    operation1(), // Start both
    operation2()  // simultaneously
  ]);
  return [result1, result2];
}