# 📚 100 Essential Java Interview Questions
#Java #Interview #Programming #OOP #Collections #Multithreading

---

## 🔹 Core Java (20 Questions)
1. What is JVM, JRE, and JDK?
2. Explain public static void main(String[] args)
3. Difference between == and .equals()?
4. What are Java primitives? List all 8.
5. Explain autoboxing/unboxing
6. What are varargs?
7. Difference between String, StringBuilder, and StringBuffer
8. What are immutable objects? How to create them?
9. Explain final, finally, and finalize
10. What are annotations? Common built-in annotations?
11. Difference between throw and throws
12. What is static keyword?
13. Can we override static methods?
14. What is method overloading vs overriding?
15. What is this and super keywords?
16. Explain pass-by-value in Java
17. What are wrapper classes? Why needed?
18. What is enum? When to use?
19. Difference between instanceof and getClass()
20. What is type casting? Implicit vs explicit

---

## 🔹 OOP Concepts (15 Questions)
21. 4 Pillars of OOP with examples
22. What is abstraction vs encapsulation?
23. Difference between abstract class and interface (Java 8+)
24. Can an interface have constructors?
25. What is polymorphism? Runtime vs compile-time
26. What is method hiding?
27. What is composition vs inheritance?
28. What is the Liskov Substitution Principle?
29. How to achieve multiple inheritance in Java?
30. What is a singleton? Thread-safe implementation
31. What is a factory pattern?
32. What is a marker interface?
33. What is a POJO?
34. What is the instanceof operator used for?
35. What is object cloning? Shallow vs deep copy

---

## 🔹 Collections Framework (15 Questions)
36. Collections hierarchy diagram explanation
37. Difference between List, Set, and Map
38. ArrayList vs LinkedList
39. HashSet vs TreeSet
40. HashMap vs HashTable vs ConcurrentHashMap
41. How HashMap works internally?
42. What is hashing? Hashcode/equals contract
43. What is Comparable vs Comparator?
44. Fail-fast vs fail-safe iterators
45. How to make collections immutable?
46. What is PriorityQueue?
47. Difference between Iterator and ListIterator
48. What are Java 8 stream APIs?
49. map() vs flatMap()
50. What are collectors? Common collectors

---

## 🔹 Exception Handling (10 Questions)
51. Exception hierarchy in Java
52. Checked vs unchecked exceptions
53. What is try-with-resources?
54. Can we have try without catch?
55. What is exception propagation?
56. Difference between throw and throws
57. How to create custom exceptions?
58. What is Error vs Exception?
59. Best practices for exception handling
60. What is @SuppressWarnings?

---

## 🔹 Multithreading (15 Questions)
61. Process vs thread
62. Ways to create threads
63. Runnable vs Callable
64. Thread lifecycle states
65. What is synchronization?
66. synchronized keyword usage
67. What are volatile variables?
68. What is deadlock? How to avoid?
69. What is thread starvation?
70. wait(), notify(), notifyAll() methods
71. What is thread pool? Executor framework
72. What is Future and CompletableFuture?
73. What is atomic variable?
74. What is ThreadLocal?
75. Concurrent collections in Java

---

## 🔹 Java 8+ Features (10 Questions)
76. What are lambda expressions?
77. Functional interfaces in Java
78. Method references types
79. Optional class purpose
80. Stream API operations
81. map() vs flatMap()
82. What are default methods?
83. What are static methods in interfaces?
84. New Date/Time API benefits
85. Records and sealed classes

---

## 🔹 JVM & Performance (10 Questions)
86. JVM architecture overview
87. What is classloader?
88. What is bytecode?
89. What is JIT compiler?
90. Heap memory structure
91. What is garbage collection? Types of GC
92. String pool concept
93. How to handle memory leaks?
94. What is OutOfMemoryError? Common causes
95. JVM tuning parameters

---

## 🔹 Advanced Topics (5 Questions)
96. What is reflection? Use cases
97. What are Java modules?
98. What is serialization? How to customize?
99. What are Java generics? Type erasure
100. What is the module system in Java 9?

---

### 📌 Bonus Tips for Interviews
✔️ Always explain with code examples
✔️ Mention real-world applications
✔️ Compare alternatives (e.g., ArrayList vs LinkedList)
✔️ Discuss performance implications
✔️ Be honest about what you don't know

#JavaInterview #CodingInterview #TechPrep 🚀

Practice these well and you'll ace any Java interview!

# 📚 JavaScript Tutorial - Part 1/10: The Complete Beginner's Guide
#JavaScript #WebDev #Programming #BeginnerFriendly

Welcome to Part 1 of our comprehensive 10-part JavaScript series! This tutorial is designed for absolute beginners with detailed explanations and practical examples.

---

## 🔹 What is JavaScript?
JavaScript is a high-level, interpreted programming language that:
- Runs in web browsers (client-side)
- Can also run on servers (Node.js)
- Adds interactivity to websites
- Works with HTML/CSS to create dynamic web pages

Key Features:
✔️ Event-driven programming
✔️ Supports object-oriented and functional styles
✔️ Dynamically typed
✔️ Asynchronous operations (callbacks, promises)

---

## 🔹 JavaScript vs Other Languages
| Feature | JavaScript | Python | Java |
|---------------|---------------|---------------|---------------|
| Typing | Dynamic | Dynamic | Static |
| Execution | Interpreted | Interpreted | Compiled |
| Platform | Browser/Server| Multi-purpose | JVM |
| Paradigms | Multi-paradigm| Multi-paradigm| OOP |

---

## 🔹 How JavaScript Runs?
1. Browser loads HTML/CSS
2. JavaScript engine (V8, SpiderMonkey) executes JS code
3. Can manipulate DOM (Document Object Model)
4. Handles user interactions

HTML → Browser → JavaScript Engine → Execution

---

## 🔹 Setting Up JavaScript
### 1. In HTML File (Most Common)

<script>
  // Your JavaScript code here
  alert("Hello World!");
</script>

<!-- OR External File -->
<script src="script.js"></script>

### 2. Browser Console
- Press F12 → Console tab
- Type JS commands directly

### 3. Node.js (Server-Side)

node filename.js

---

## 🔹 Your First JavaScript Program

// Single line comment
/* Multi-line 
   comment */

// Print to console
console.log("Hello World!"); 

// Alert popup
alert("Welcome to JavaScript!");

// HTML output
document.write("<h1>Hello from JS!</h1>");

---

## 🔹 Variables & Data Types
JavaScript has 3 ways to declare variables:

### 1. Variable Declaration

let age = 25;        // Mutable (block-scoped)
const PI = 3.14;     // Immutable 
var name = "Ali";    // Old way (function-scoped)

### 2. Data Types
| Type | Example | Description |
|-------------|--------------------------|--------------------------|
| Number | 42, 3.14 | All numbers |
| String | "Hello", 'World' | Text data |
| Boolean | true, false | Logical values |
| Object | {name: "Ali", age: 25} | Key-value pairs |
| Array | [1, 2, 3] | Ordered lists |
| Null | null | Intentional empty value |
| Undefined | undefined | Uninitialized variable |

### 3. Type Checking

typeof "Hello";    // "string"
typeof 42;         // "number"
typeof true;       // "boolean"
typeof {};         // "object"

---

## 🔹 Operators
### 1. Arithmetic

let x = 10, y = 3;
console.log(x + y);  // 13
console.log(x - y);  // 7
console.log(x * y);  // 30
console.log(x / y);  // 3.333...
console.log(x % y);  // 1 (modulus)

### 2. Comparison

console.log(5 == "5");   // true (loose equality)
console.log(5 === "5");  // false (strict equality)
console.log(5 != "5");   // false
console.log(5 !== "5");  // true

### 3. Logical

true && false;    // AND → false
true || false;    // OR → true
!true;            // NOT → false

---

## 🔹 Type Conversion
### 1. Explicit Conversion

String(123);        // "123"
Number("3.14");     // 3.14
Boolean(1);         // true

### 2. Implicit Conversion

"5" + 2;      // "52" (string concatenation)
"5" - 2;      // 3 (numeric operation)

---

## 🔹 Practical Example: Simple Calculator

<script>
  let num1 = parseFloat(prompt("Enter first number:"));
  let num2 = parseFloat(prompt("Enter second number:"));
  
  console.log(`Addition: ${num1 + num2}`);
  console.log(`Subtraction: ${num1 - num2}`);
  console.log(`Multiplication: ${num1 * num2}`);
  console.log(`Division: ${num1 / num2}`);
</script>

---

## 🔹 Best Practices for Beginners
1. Use `const` by default, let when needed, avoid var
2. Always declare variables before use
3. Use strict equality (`===`) instead of ==
4. Name variables meaningfully (e.g., userAge not x)
5. Comment your code for complex logic

---

### 📌 What's Next?
In Part 2, we'll cover:
➡️ Conditionals (if/else, switch)
➡️ Loops (for, while)
➡️ Functions

#LearnJavaScript #CodingBasics #WebDevelopment 🚀

Practice Exercise:
1. Create variables for your name, age, and country
2. Calculate the area of a circle (PI * r²)
3. Try different type conversions

# 📚 JavaScript Tutorial - Part 2/10: Control Flow & Functions
#JavaScript #WebDev #Programming #Beginners

Welcome to Part 2 of our JavaScript series! Today we'll master decision-making and functions - the building blocks of programming logic.

---

## 🔹 Conditional Statements
Control program flow based on conditions.

### 1. if-else Statement

let age = 18;

if (age >= 18) {
    console.log("You're an adult");
} else {
    console.log("You're a minor");
}

### 2. Ternary Operator (Shorthand if-else)

let message = (age >= 18) ? "Adult" : "Minor";
console.log(message);

### 3. switch-case Statement

let day = 3;
let dayName;

switch(day) {
    case 1: dayName = "Monday"; break;
    case 2: dayName = "Tuesday"; break;
    case 3: dayName = "Wednesday"; break;
    default: dayName = "Unknown";
}

console.log(dayName); // "Wednesday"

---

## 🔹 Loops
Execute code repeatedly.

### 1. for Loop

for (let i = 1; i <= 5; i++) {
    console.log(`Count: ${i}`);
}
// Output: 1, 2, 3, 4, 5

### 2. while Loop

let count = 1;
while (count <= 5) {
    console.log(count);
    count++;
}

### 3. do-while Loop

let x = 1;
do {
    console.log(x);
    x++;
} while (x <= 5);

### 4. for...of Loop (Arrays)

const colors = ["red", "green", "blue"];
for (const color of colors) {
    console.log(color);
}

### 5. for...in Loop (Objects)

const person = {name: "Ali", age: 25};
for (const key in person) {
    console.log(`${key}: ${person[key]}`);
}

---

## 🔹 Functions
Reusable blocks of code.

### 1. Function Declaration

function greet(name) {
    return `Hello, ${name}!`;
}
console.log(greet("Ali")); // "Hello, Ali!"

### 2. Function Expression

const square = function(x) {
    return x * x;
};
console.log(square(5)); // 25

### 3. Arrow Functions (ES6+)

const add = (a, b) => a + b;
console.log(add(2, 3)); // 5

### 4. Default Parameters

function createUser(name, role = "user") {
    console.log(`${name} is a ${role}`);
}
createUser("Ali"); // "Ali is a user"

### 5. Rest Parameters

function sum(...numbers) {
    return numbers.reduce((total, num) => total + num, 0);
}
console.log(sum(1, 2, 3)); // 6

---

## 🔹 Practical Example: Grade Calculator

function calculateGrade(score) {
    if (score >= 90) return "A";
    else if (score >= 80) return "B";
    else if (score >= 70) return "C";
    else return "F";
}

const studentScore = 85;
console.log(`Grade: ${calculateGrade(studentScore)}`); // "Grade: B"

---

## 🔹 Scope in JavaScript
### 1. Global Scope

const globalVar = "I'm global";

function test() {
    console.log(globalVar); // Accessible
}

### 2. Function Scope (var)

function test() {
    var functionVar = "I'm function-scoped";
}
console.log(functionVar); // Error: Not accessible

### 3. Block Scope (let/const)

if (true) {
    let blockVar = "I'm block-scoped";
}
console.log(blockVar); // Error: Not accessible

---

## 🔹 Error Handling
### 1. try-catch-finally

try {
    // Risky code
    nonExistentFunction();
} catch (error) {
    console.log("Error occurred:", error.message);
} finally {
    console.log("This always executes");
}

### 2. Throwing Custom Errors

function divide(a, b) {
    if (b === 0) throw new Error("Cannot divide by zero!");
    return a / b;
}

---

## 🔹 Best Practices
1. Use strict equality (===) over ==
2. Prefer const/let over var
3. Keep functions small/single-purpose
4. Always handle errors
5. Use descriptive names for functions/variables

---

### 📌 What's Next?
In Part 3, we'll cover:
➡️ Arrays & Array Methods
➡️ Objects & JSON
➡️ Destructuring

#LearnJavaScript #CodingBasics #WebDevelopment 🚀

Practice Exercise:
1. Create a function to check if a number is even/odd
2. Write a loop that prints prime numbers up to 20
3. Make a temperature converter function (Celsius ↔️ Fahrenheit)

# 📚 JavaScript Tutorial - Part 3/10: Arrays, Objects & JSON
#JavaScript #WebDev #Programming #DataStructures

Welcome to Part 3 of our JavaScript series! Today we'll master arrays, objects, and JSON - essential for handling complex data.

---

## 🔹 Arrays in JavaScript
Ordered collections that can hold multiple values.

### 1. Creating Arrays

// Array literal (recommended)
const fruits = ['Apple', 'Banana', 'Orange'];

// Array constructor
const numbers = new Array(1, 2, 3);

// Mixed data types
const mixed = [1, 'Hello', true, {name: 'Ali'}];

### 2. Accessing Elements

console.log(fruits[0]);  // "Apple" (0-based index)
console.log(fruits.length);  // 3

### 3. Common Array Methods
| Method | Description | Example |
|--------|-------------|---------|
| push() | Add to end | fruits.push('Mango') |
| pop() | Remove from end | fruits.pop() |
| shift() | Remove from start | fruits.shift() |
| unshift() | Add to start | fruits.unshift('Kiwi') |
| slice() | Get sub-array | fruits.slice(1, 3) |
| splice() | Add/remove items | fruits.splice(1, 0, 'Berry') |

---

## 🔹 Modern Array Methods (ES6+)
Powerful functional programming tools.

### 1. forEach()

fruits.forEach(fruit => console.log(fruit));

### 2. map()

const upperFruits = fruits.map(fruit => fruit.toUpperCase());

### 3. filter()

const longFruits = fruits.filter(fruit => fruit.length > 5);

### 4. reduce()

const sum = [1, 2, 3].reduce((total, num) => total + num, 0);

### 5. find() & findIndex()

const banana = fruits.find(fruit => fruit === 'Banana');
const bananaIndex = fruits.findIndex(fruit => fruit === 'Banana');

### 6. some() & every()

const hasApple = fruits.some(fruit => fruit === 'Apple');
const allLong = fruits.every(fruit => fruit.length > 3);

---

## 🔹 Objects in JavaScript
Collections of key-value pairs (properties).

### 1. Creating Objects

// Object literal (recommended)
const person = {
    name: 'Ali',
    age: 25,
    hobbies: ['reading', 'coding'],
    address: {
        city: 'Dubai',
        country: 'UAE'
    }
};

### 2. Accessing Properties

// Dot notation
console.log(person.name);  // "Ali"

// Bracket notation (useful for dynamic keys)
const key = 'age';
console.log(person[key]);  // 25

### 3. Modifying Objects

// Add property
person.job = 'Developer';

// Delete property
delete person.age;

// Check property existence
'name' in person;  // true

---

## 🔹 Object Methods
### 1. Object.keys()

const keys = Object.keys(person);  // ["name", "age", ...]

### 2. Object.values()

const values = Object.values(person);

### 3. Object.entries()

for (const [key, value] of Object.entries(person)) {
    console.log(`${key}: ${value}`);
}

### 4. Object Spread (ES9+)

const newPerson = {...person, age: 26};

---

## 🔹 JSON (JavaScript Object Notation)
Universal data format for APIs.

### 1. JSON ↔️ JavaScript

// Object to JSON string
const jsonStr = JSON.stringify(person);

// JSON string to object
const parsedObj = JSON.parse(jsonStr);

### 2. JSON Structure

{
    "name": "Ali",
    "age": 25,
    "isStudent": false,
    "courses": ["Math", "CS"]
}

---

## 🔹 Destructuring Assignment
Unpack values from arrays/objects.

### 1. Array Destructuring

const [first, second] = fruits;
console.log(first);  // "Apple"

### 2. Object Destructuring

const {name, age} = person;
console.log(name);  // "Ali"

// With renaming
const {name: personName} = person;

### 3. Function Parameter Destructuring

function printUser({name, age}) {
    console.log(`${name} is ${age} years old`);
}

---

## 🔹 Practical Example: Todo List Manager

const todoList = [
    {id: 1, task: 'Buy groceries', completed: false},
    {id: 2, task: 'Do laundry', completed: true}
];

// Add new task
function addTask(task) {
    const newTask = {
        id: todoList.length + 1,
        task,
        completed: false
    };
    todoList.push(newTask);
}

// Mark task as complete
function completeTask(id) {
    const task = todoList.find(item => item.id === id);
    if (task) task.completed = true;
}

// Get pending tasks
function getPendingTasks() {
    return todoList.filter(task => !task.completed);
}

---

## 🔹 Best Practices
1. Use `const` for arrays/objects (the reference is constant)
2. Prefer functional methods (map, filter) over loops
3. Validate JSON before parsing
4. Use descriptive property names
5. Destructure deeply nested objects carefully

---

### 📌 What's Next?
In Part 4, we'll cover:
➡️ DOM Manipulation
➡️ Event Handling
➡️ Form Validation

#JavaScript #WebDevelopment #Programming 🚀

Practice Exercise:
1. Create an array of products (name, price, inStock)
2. Write functions to:
- Filter out-of-stock products
- Calculate total inventory value
- Sort by price (high-to-low)
3. Convert your array to JSON and back

# 📚 JavaScript Tutorial - Part 4/10: DOM Manipulation & Events
#JavaScript #WebDev #FrontEnd #DOM

Welcome to Part 4 of our JavaScript series! Today we'll learn how to make web pages interactive by mastering DOM manipulation and event handling.

---

## 🔹 What is the DOM?
The Document Object Model is a tree-like representation of your webpage that JavaScript can interact with.

<!-- HTML Structure -->
<html>
  <head>
    <title>My Page</title>
  </head>
  <body>
    <h1 id="main-title">Hello World</h1>
    <ul class="items">
      <li>Item 1</li>
      <li>Item 2</li>
    </ul>
  </body>
</html>

graph TD
  document --> html
  html --> head
  html --> body
  head --> title
  title --> text["My Page"]
  body --> h1
  h1 --> h1text["Hello World"]
  body --> ul
  ul --> li1["Item 1"]
  ul --> li2["Item 2"]

---

## 🔹 Selecting DOM Elements
### 1. Single Element Selectors

// By ID (returns single element)
const title = document.getElementById('main-title');

// By CSS selector (first match)
const item = document.querySelector('.items li');

### 2. Multiple Element Selectors

// By class name (HTMLCollection)
const items = document.getElementsByClassName('item');

// By tag name (HTMLCollection)
const listItems = document.getElementsByTagName('li');

// By CSS selector (NodeList)
const allItems = document.querySelectorAll('.items li');

---

## 🔹 Modifying the DOM
### 1. Changing Content

// Text content (better for security)
title.textContent = 'New Title';

// HTML content (use carefully!)
title.innerHTML = '<em>New</em> Title';

// Attribute changes
title.setAttribute('class', 'highlight');

### 2. Styling Elements

// Direct style property access
title.style.color = 'blue';
title.style.fontSize = '2rem';

// Multiple styles at once
title.style.cssText = 'color: blue; font-size: 2rem;';

// Toggle classes
title.classList.add('highlight');
title.classList.remove('highlight');
title.classList.toggle('highlight');

### 3. Creating & Adding Elements

// Create new element
const newItem = document.createElement('li');
newItem.textContent = 'Item 3';

// Append to parent
const list = document.querySelector('ul');
list.appendChild(newItem);

// Insert at specific position
list.insertBefore(newItem, list.children[1]);

// Remove elements
list.removeChild(newItem);

---

## 🔹 Event Handling
Responding to user interactions.

### 1. Inline Events (Avoid)

<button onclick="handleClick()">Click Me</button>

### 2. Recommended Approach

const button = document.querySelector('button');

// Add event listener
button.addEventListener('click', function(e) {
  console.log('Button clicked!', e.target);
});

// Remove event listener
button.removeEventListener('click', handleClick);

### 3. Common Event Types
| Event | Description |
|-------|-------------|
| click | Mouse click |
| dblclick | Double click |
| mouseenter/mouseleave | Hover effects |
| keydown/keyup | Keyboard input |
| submit | Form submission |
| load | Page/images loaded |
| scroll | Page scrolling |

---

## 🔹 Event Object & Propagation
### 1. Event Object Properties

element.addEventListener('click', function(event) {
  console.log(event.type);      // "click"
  console.log(event.target);    // Clicked element
  console.log(event.clientX);   // Mouse X position
  console.log(event.key);       // Key pressed (for keyboard events)
});

### 2. Stopping Propagation

// Stop bubbling up
event.stopPropagation();

// Prevent default behavior
event.preventDefault();

### 3. Event Delegation

document.querySelector('ul').addEventListener('click', function(e) {
  if (e.target.tagName === 'LI') {
    console.log('List item clicked:', e.target.textContent);
  }
});

---

## 🔹 Practical Example: Interactive Todo List

// DOM Elements
const form = document.querySelector('#todo-form');
const input = document.querySelector('#todo-input');
const list = document.querySelector('#todo-list');

// Add new todo
form.addEventListener('submit', function(e) {
  e.preventDefault();
  
  if (input.value.trim() === '') return;
  
  const todoText = input.value;
  const li = document.createElement('li');
  li.innerHTML = `
    ${todoText}
    <button class="delete-btn">X</button>
  `;
  
  list.appendChild(li);
  input.value = '';
});

// Delete todo (using delegation)
list.addEventListener('click', function(e) {
  if (e.target.classList.contains('delete-btn')) {
    e.target.parentElement.remove();
  }
});

---

## 🔹 Working with Forms
### 1. Accessing Form Data

const form = document.querySelector('form');
form.addEventListener('submit', function(e) {
  e.preventDefault();
  
  // Get form values
  const username = form.elements['username'].value;
  const password = form.elements['password'].value;
  
  console.log({ username, password });
});

### 2. Form Validation

function validateForm() {
  const email = document.getElementById('email').value;
  
  if (!email.includes('@')) {
    alert('Please enter a valid email');
    return false;
  }
  
  return true;
}

---

## 🔹 Best Practices
1. Cache DOM queries (store in variables)
2. Use event delegation for dynamic elements
3. Always prevent default on form submissions
4. Separate JS from HTML (avoid inline handlers)
5. Throttle rapid-fire events (resize, scroll)

---

### 📌 What's Next?
In Part 5, we'll cover:
➡️ Asynchronous JavaScript
➡️ Callbacks, Promises, Async/Await
➡️ Fetch API & AJAX

#JavaScript #FrontEnd #WebDevelopment 🚀

Practice Exercise:
1. Create a color picker that changes background color
2. Build a counter with + and - buttons
3. Make a dropdown menu that shows/hides on click

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# 📚 JavaScript Tutorial - Part 5/10: Asynchronous JavaScript
#JavaScript #Async #Promises #FetchAPI #WebDev

Welcome to Part 5 of our JavaScript series! Today we'll conquer asynchronous programming - the key to handling delays, API calls, and non-blocking operations.

---

## 🔹 Synchronous vs Asynchronous
### 1. Synchronous Execution

console.log("Step 1");
console.log("Step 2"); // Waits for Step 1
console.log("Step 3"); // Waits for Step 2

### 2. Asynchronous Execution

console.log("Start");

setTimeout(() => {
  console.log("Async operation complete");
}, 2000);

console.log("End");

// Output order: Start → End → Async operation complete

---

## 🔹 Callback Functions
Traditional way to handle async operations.

### 1. Basic Callback

function fetchData(callback) {
  setTimeout(() => {
    callback("Data received");
  }, 1000);
}

fetchData((data) => {
  console.log(data); // "Data received" after 1 second
});

### 2. Callback Hell (The Pyramid of Doom)

getUser(user => {
  getPosts(user.id, posts => {
    getComments(posts[0].id, comments => {
      console.log(comments); // Nested nightmare!
    });
  });
});

---

## 🔹 Promises (ES6)
Modern solution for async operations.

### 1. Promise States
- Pending: Initial state
- Fulfilled: Operation completed successfully
- Rejected: Operation failed

### 2. Creating Promises

const fetchData = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = true;
    success ? resolve("Data fetched!") : reject("Error!");
  }, 1500);
});

### 3. Using Promises

fetchData
  .then(data => console.log(data))
  .catch(error => console.error(error))
  .finally(() => console.log("Done!"));

### 4. Promise Chaining

fetchUser()
  .then(user => fetchPosts(user.id))
  .then(posts => fetchComments(posts[0].id))
  .then(comments => console.log(comments))
  .catch(error => console.error(error));

### 5. Promise Methods

Promise.all([promise1, promise2]) // Waits for all
Promise.race([promise1, promise2]) // First to settle
Promise.any([promise1, promise2]) // First to fulfill

---

## 🔹 Async/Await (ES8)
Syntactic sugar for promises.

### 1. Basic Usage

async function getData() {
  try {
    const response = await fetchData();
    console.log(response);
  } catch (error) {
    console.error(error);
  }
}

### 2. Parallel Execution

async function fetchAll() {
  const [users, posts] = await Promise.all([
    fetchUsers(),
    fetchPosts()
  ]);
  console.log(users, posts);
}

---

## 🔹 Fetch API
Modern way to make HTTP requests.

### 1. GET Request

async function getUsers() {
  const response = await fetch('https://api.example.com/users');
  const data = await response.json();
  return data;
}

### 2. POST Request

async function createUser(user) {
  const response = await fetch('https://api.example.com/users', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    body: JSON.stringify(user)
  });
  return response.json();
}

### 3. Error Handling

async function safeFetch(url) {
  try {
    const response = await fetch(url);
    if (!response.ok) throw new Error(response.status);
    return await response.json();
  } catch (error) {
    console.error("Fetch failed:", error);
  }
}

---

## 🔹 Practical Example: Weather App

async function getWeather(city) {
  try {
    const apiKey = 'YOUR_API_KEY';
    const response = await fetch(
      `https://api.openweathermap.org/data/2.5/weather?q=${city}&appid=${apiKey}`
    );
    
    if (!response.ok) throw new Error('City not found');
    
    const data = await response.json();
    return {
      temp: Math.round(data.main.temp - 273.15), // Kelvin → Celsius
      conditions: data.weather[0].main
    };
  } catch (error) {
    console.error("Weather fetch error:", error);
    return null;
  }
}

// Usage
const weather = await getWeather("London");
if (weather) {
  console.log(`Temperature: ${weather.temp}°C`);
  console.log(`Conditions: ${weather.conditions}`);
}

---

## 🔹 AJAX with XMLHttpRequest (Legacy)
Older way to make requests (still good to know).

function oldFetch(url, callback) {
  const xhr = new XMLHttpRequest();
  xhr.open('GET', url);
  xhr.onload = () => {
    if (xhr.status === 200) {
      callback(JSON.parse(xhr.response));
    } else {
      callback(null, xhr.status);
    }
  };
  xhr.send();
}

---

## 🔹 Best Practices
1. Always handle errors in promises/async functions
2. Use async/await for better readability
3. Cancel requests when no longer needed (AbortController)
4. Throttle rapid API calls (debounce input handlers)
5. Cache responses when appropriate

---

### 📌 What's Next?
In Part 6, we'll cover:
➡️ JavaScript Modules
➡️ ES6+ Features
➡️ Tooling (Babel, Webpack, npm)

#JavaScript #AsyncProgramming #WebDevelopment 🚀

Practice Exercise:
1. Fetch GitHub user data (https://api.github.com/users/username)
2. Create a function that fetches multiple Pokémon in parallel
3. Build a retry mechanism for failed requests (max 3 attempts)

# 📚 JavaScript Tutorial - Part 6/10: Modules & Modern JavaScript
#JavaScript #ES6 #Modules #Webpack #ModernJS

Welcome to Part 6 of our JavaScript series! Today we'll explore code organization with modules and modern JavaScript features that revolutionized frontend development.

---

## 🔹 JavaScript Modules
### 1. Module Systems Comparison
| System | Syntax | Environment | Features |
|--------------|-----------------|--------------|----------|
| ES Modules | import/export | Modern browsers, Node.js | Native standard |
| CommonJS | require/module.exports | Node.js | Legacy Node |
| AMD | define/require | Browser | Async loading |

### 2. ES Modules (ES6)
#### Exporting:

// Named exports (multiple per file)
export const API_URL = 'https://api.example.com';
export function fetchData() { /* ... */ }

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

#### Importing:

// Named imports
import { API_URL, fetchData } from './utils.js';

// Default import
import User from './models/User.js';

// Mixed imports
import User, { API_URL } from './config.js';

// Import everything
import * as utils from './utils.js';

### 3. HTML Integration

<script type="module">
  import { initApp } from './app.js';
  initApp();
</script>

---

## 🔹 Modern JavaScript Features
### 1. Block-Scoped Declarations (ES6)

let x = 10;    // Block-scoped variable
const PI = 3.14; // Block-scoped constant

### 2. Template Literals (ES6)

const name = 'Ali';
console.log(`Hello ${name}! Today is ${new Date().toLocaleDateString()}`);

### 3. Arrow Functions (ES6)

const add = (a, b) => a + b;
[1, 2, 3].map(n => n * 2);

### 4. Destructuring (ES6)

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

// Object destructuring
const { name, age } = user;

### 5. Spread/Rest Operator (ES6)

// Spread
const nums = [1, 2, 3];
const newNums = [...nums, 4, 5];

// Rest parameters
function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}

### 6. Optional Chaining (ES2020)

const street = user?.address?.street; // No error if null

### 7. Nullish Coalescing (ES2020)

const limit = config.maxItems ?? 10; // Only if null/undefined

---

## 🔹 JavaScript Tooling
### 1. Package Management with npm/yarn

npm init -y                 # Initialize project
npm install lodash          # Install package
npm install --save-dev webpack # Dev dependency

### 2. Module Bundlers
#### Webpack Configuration (webpack.config.js):

module.exports = {
  entry: './src/index.js',
  output: {
    filename: 'bundle.js',
    path: path.resolve(__dirname, 'dist')
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: 'babel-loader'
      }
    ]
  }
};

### 3. Babel (JavaScript Compiler)
#### .babelrc Configuration:

{
  "presets": ["@babel/preset-env"],
  "plugins": ["@babel/plugin-transform-runtime"]
}

### 4. ESLint (Code Linter)
#### .eslintrc.json Example:

{
  "extends": "eslint:recommended",
  "rules": {
    "semi": ["error", "always"],
    "quotes": ["error", "single"]
  }
}

---

## 🔹 Practical Example: Modular App Structure

project/
├── src/
│   ├── index.js          # Entry point
│   ├── utils/            # Helper modules
│   │   ├── api.js        # API functions
│   │   └── dom.js        # DOM helpers
│   ├── components/       # UI components
│   │   ├── header.js
│   │   └── modal.js
│   └── styles/
│       └── main.css      # Imported in JS
├── package.json          # npm config
├── webpack.config.js     # Bundler config
└── .babelrc              # Compiler config

Example Component (`components/header.js`):

export function createHeader(title) {
  const header = document.createElement('header');
  header.innerHTML = `<h1>${title}</h1>`;
  return header;
}

Main Entry Point (`index.js`):

import { createHeader } from './components/header.js';
import { fetchPosts } from './utils/api.js';

document.body.appendChild(createHeader('My Blog'));

async function init() {
  const posts = await fetchPosts();
  console.log('Loaded posts:', posts);
}

init();

---

## 🔹 Modern Browser APIs
### 1. Web Components

class MyElement extends HTMLElement {
  connectedCallback() {
    this.innerHTML = `<h2>Custom Element</h2>`;
  }
}

customElements.define('my-element', MyElement);

### 2. Intersection Observer

const observer = new IntersectionObserver((entries) => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      entry.target.classList.add('visible');
    }
  });
});

document.querySelectorAll('.animate').forEach(el => {
  observer.observe(el);
});

### 3. Web Storage

// Local Storage (persistent)
localStorage.setItem('theme', 'dark');
const theme = localStorage.getItem('theme');

// Session Storage (tab-specific)
sessionStorage.setItem('token', 'abc123');

---

## 🔹 Best Practices
1. Use ES modules for modern projects
2. Keep modules focused (single responsibility)
3. Configure tree-shaking to eliminate dead code
4. Use semantic versioning (^1.2.3 in package.json)
5. Set up pre-commit hooks (linting, formatting)

---

### 📌 What's Next?
In Part 7, we'll cover:
➡️ Object-Oriented JavaScript
➡️ Classes & Prototypes
➡️ Design Patterns

#JavaScript #ModernWeb #FrontendDevelopment 🚀

Practice Exercise:
1. Convert an existing script to ES modules
2. Create a Webpack config that bundles JS and CSS
3. Build a custom element with shadow DOM

# 📚 JavaScript Tutorial - Part 7/10: Object-Oriented JavaScript & Prototypes
#JavaScript #OOP #Prototypes #Classes #DesignPatterns

Welcome to Part 7 of our JavaScript series! This comprehensive lesson will take you deep into JavaScript's object-oriented programming (OOP) system, prototypes, classes, and design patterns.

---

## 🔹 JavaScript OOP Fundamentals
### 1. Objects in JavaScript
JavaScript objects are dynamic collections of properties with a hidden [[Prototype]] property.

// Object literal (most common)
const person = {
  name: 'Ali',
  age: 25,
  greet() {
    console.log(`Hello, I'm ${this.name}`);
  }
};

// Properties can be added dynamically
person.country = 'UAE';
delete person.age;

### 2. Factory Functions
Functions that create and return objects.

function createPerson(name, age) {
  return {
    name,
    age,
    greet() {
      console.log(`Hi, I'm ${this.name}`);
    }
  };
}

const ali = createPerson('Ali', 25);

### 3. Constructor Functions
The traditional way to create object blueprints.

function Person(name, age) {
  // Instance properties
  this.name = name;
  this.age = age;
  
  // Method (created for each instance)
  this.greet = function() {
    console.log(`Hello, I'm ${this.name}`);
  };
}

const ali = new Person('Ali', 25);

The `new` keyword does:
1. Creates a new empty object
2. Sets this to point to the new object
3. Links the object's prototype to constructor's prototype
4. Returns the object (unless constructor returns something else)

---

## 🔹 Prototypes & Inheritance
### 1. Prototype Chain
Every JavaScript object has a [[Prototype]] link to another object.

// Adding to prototype (shared across instances)
Person.prototype.introduce = function() {
  console.log(`My name is ${this.name}, age ${this.age}`);
};

// Prototype chain lookup
ali.introduce(); // Checks ali → Person.prototype → Object.prototype → null

### 2. Manual Prototype Inheritance

function Student(name, age, grade) {
  Person.call(this, name, age); // "Super" constructor
  this.grade = grade;
}

// Set up prototype chain
Student.prototype = Object.create(Person.prototype);
Student.prototype.constructor = Student;

// Add methods
Student.prototype.study = function() {
  console.log(`${this.name} is studying hard!`);
};

### 3. Object.create()
Pure prototypal inheritance.

const personProto = {
  greet() {
    console.log(`Hello from ${this.name}`);
  }
};

const ali = Object.create(personProto);
ali.name = 'Ali';

---

## 🔹 ES6 Classes
Syntactic sugar over prototypes.

### 1. Class Syntax

class Person {
  // Constructor (called with 'new')
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
  
  // Instance method
  greet() {
    console.log(`Hello, I'm ${this.name}`);
  }
  
  // Static method
  static compareAges(a, b) {
    return a.age - b.age;
  }
}

### 2. Inheritance with `extends`

class Student extends Person {
  constructor(name, age, grade) {
    super(name, age); // Must call super first
    this.grade = grade;
  }
  
  study() {
    console.log(`${this.name} is studying`);
  }
  
  // Override method
  greet() {
    super.greet(); // Call parent method
    console.log(`I'm in grade ${this.grade}`);
  }
}

### 3. Class Features (ES2022+)

class ModernClass {
  // Public field (instance property)
  publicField = 1;
  
  // Private field (starts with #)
  #privateField = 2;
  
  // Static public field
  static staticField = 3;
  
  // Static private field
  static #staticPrivateField = 4;
  
  // Private method
  #privateMethod() {
    return this.#privateField;
  }
}

---

## 🔹 Property Descriptors
Advanced control over object properties.

### 1. Property Attributes

const obj = {};

Object.defineProperty(obj, 'readOnlyProp', {
  value: 42,
  writable: false,       // Can't be changed
  enumerable: true,      // Shows up in for...in
  configurable: false    // Can't be deleted/reconfigured
});

### 2. Getters & Setters

class Temperature {
  constructor(celsius) {
    this.celsius = celsius;
  }
  
  get fahrenheit() {
    return this.celsius * 1.8 + 32;
  }
  
  set fahrenheit(value) {
    this.celsius = (value - 32) / 1.8;
  }
}

const temp = new Temperature(25);
console.log(temp.fahrenheit); // 77
temp.fahrenheit = 100;

---

## 🔹 Design Patterns in JavaScript
### 1. Singleton Pattern

class AppConfig {
  constructor() {
    if (AppConfig.instance) {
      return AppConfig.instance;
    }
    
    this.settings = { theme: 'dark' };
    AppConfig.instance = this;
  }
}

const config1 = new AppConfig();
const config2 = new AppConfig();
console.log(config1 === config2); // true

### 2. Factory Pattern

class UserFactory {
  static createUser(type) {
    switch(type) {
      case 'admin':
        return new Admin();
      case 'customer':
        return new Customer();
      default:
        throw new Error('Invalid user type');
    }
  }
}

### 3. Observer Pattern

class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, listener) {
    (this.events[event] || (this.events[event] = [])).push(listener);
  }
  
  emit(event, ...args) {
    this.events[event]?.forEach(listener => listener(...args));
  }
}

const emitter = new EventEmitter();
emitter.on('login', user => console.log(`${user} logged in`));
emitter.emit('login', 'Ali');

### 4. Module Pattern

const CounterModule = (() => {
  let count = 0;
  
  return {
    increment() {
      count++;
    },
    getCount() {
      return count;
    }
  };
})();

---

## 🔹 Practical Example: RPG Character System

class Character {
  constructor(name, level) {
    this.name = name;
    this.level = level;
    this.health = 100;
  }
  
  attack(target) {
    const damage = this.level * 2;
    target.takeDamage(damage);
    console.log(`${this.name} attacks ${target.name} for ${damage} damage`);
  }
  
  takeDamage(amount) {
    this.health -= amount;
    if (this.health <= 0) {
      console.log(`${this.name} has been defeated!`);
    }
  }
}

class Mage extends Character {
  constructor(name, level, mana) {
    super(name, level);
    this.mana = mana;
  }
  
  castSpell(target) {
    if (this.mana >= 10) {
      const damage = this.level * 3;
      this.mana -= 10;
      target.takeDamage(damage);
      console.log(`${this.name} casts a spell on ${target.name}!`);
    } else {
      console.log("Not enough mana!");
    }
  }
}

// Usage
const warrior = new Character('Conan', 5);
const wizard = new Mage('Gandalf', 7, 50);

warrior.attack(wizard);
wizard.castSpell(warrior);

---

## 🔹 Performance Considerations
### 1. Prototype vs Instance Methods
- Prototype methods are memory efficient (shared)
- Instance methods are created per object

### 2. Object Creation Patterns
| Pattern | Speed | Memory | Features |
|---------|-------|--------|----------|
| Constructor | Fast | Efficient | Full prototype chain |
| Factory | Medium | Less efficient | No instanceof |
| Class | Fast | Efficient | Clean syntax |

### 3. Property Access Optimization

// Faster (direct property access)
obj.propertyName;

// Slower (dynamic property access)
obj['property' + 'Name'];

---

## 🔹 Best Practices
1. Use classes for complex hierarchies
2. Favor composition over deep inheritance
3. Keep prototypes lean for performance
4. Use private fields for encapsulation
5. Document your classes with JSDoc

---

### 📌 What's Next?
In Part 8, we'll cover:
➡️ Functional Programming in JavaScript
➡️ Pure Functions & Immutability
➡️ Higher-Order Functions
➡️ Redux Patterns

#JavaScript #OOP #DesignPatterns 🚀

Practice Exercise:
1. Implement a Vehicle → Car → ElectricCar hierarchy
2. Create a BankAccount class with private balance
3. Build an observable ShoppingCart using the Observer pattern

# 📚 JavaScript Tutorial - Part 8/10: Functional Programming in JavaScript
#JavaScript #FunctionalProgramming #FP #PureFunctions #HigherOrderFunctions

Welcome to Part 8 of our JavaScript series! Today we'll explore functional programming (FP) concepts that will transform how you write JavaScript, making your code more predictable, reusable, and maintainable.

---

## 🔹 Core Principles of Functional Programming
### 1. Pure Functions

// Pure function (same input → same output, no side effects)
function add(a, b) {
  return a + b;
}

// Impure function (side effect + depends on external state)
let taxRate = 0.1;
function calculateTax(amount) {
  return amount * taxRate; // Depends on external variable
}

### 2. Immutability

// Bad (mutates original array)
const addToCart = (cart, item) => {
  cart.push(item); // Mutation!
  return cart;
};

// Good (returns new array)
const addToCartFP = (cart, item) => [...cart, item];

### 3. Function Composition

const compose = (...fns) => x => fns.reduceRight((v, f) => f(v), x);

const toUpperCase = str => str.toUpperCase();
const exclaim = str => `${str}!`;
const shout = compose(exclaim, toUpperCase);

shout('hello'); // "HELLO!"

---

## 🔹 First-Class Functions
### 1. Functions as Arguments

const numbers = [1, 2, 3];

// Passing function as argument
numbers.map(num => num * 2); // [2, 4, 6]

### 2. Returning Functions

const createMultiplier = factor => num => num * factor;
const double = createMultiplier(2);
double(5); // 10

### 3. Storing Functions

const mathOps = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b
};
mathOps.add(3, 5); // 8

---

## 🔹 Higher-Order Functions
### 1. Array Methods

const products = [
  { id: 1, name: 'Laptop', price: 999, inStock: true },
  { id: 2, name: 'Mouse', price: 25, inStock: false }
];

// Transform data
const productNames = products.map(p => p.name);

// Filter data
const availableProducts = products.filter(p => p.inStock);

// Reduce to single value
const totalPrice = products.reduce((sum, p) => sum + p.price, 0);

### 2. Custom HOF Example

function withLogging(fn) {
  return (...args) => {
    console.log(`Calling with args: ${args}`);
    const result = fn(...args);
    console.log(`Result: ${result}`);
    return result;
  };
}

const loggedAdd = withLogging(add);
loggedAdd(3, 5);

---

## 🔹 Closures
Functions that remember their lexical scope.

### 1. Basic Closure

function createCounter() {
  let count = 0;
  return () => ++count;
}

const counter = createCounter();
counter(); // 1
counter(); // 2

### 2. Practical Use Case

function createApiClient(baseUrl) {
  return {
    get(endpoint) {
      return fetch(`${baseUrl}${endpoint}`).then(res => res.json());
    },
    post(endpoint, data) {
      return fetch(`${baseUrl}${endpoint}`, {
        method: 'POST',
        body: JSON.stringify(data)
      });
    }
  };
}

const jsonPlaceholder = createApiClient('https://jsonplaceholder.typicode.com');
jsonPlaceholder.get('/posts/1').then(console.log);

---

## 🔹 Currying
Transforming a multi-argument function into a sequence of single-argument functions.

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

// Curried version
const curriedAdd = a => b => c => a + b + c;
curriedAdd(1)(2)(3); // 6

// Practical example
const createUser = username => email => password => ({
  username,
  email,
  password
});

const registerUser = createUser('js_dev');
const withEmail = registerUser('[email protected]');
const finalUser = withEmail('secure123');

---

## 🔹 Recursion
### 1. Basic Recursion

function factorial(n) {
  return n <= 1 ? 1 : n * factorial(n - 1);
}

### 2. Tail Call Optimization

function factorial(n, acc = 1) {
  return n <= 1 ? acc : factorial(n - 1, n * acc);
}

### 3. Recursive Array Processing

function deepMap(arr, fn) {
  return arr.map(item => 
    Array.isArray(item) ? deepMap(item, fn) : fn(item)
  );
}

deepMap([1, [2, [3]]], x => x * 2); // [2, [4, [6]]]

---

## 🔹 Practical Example: Redux-like Store

function createStore(reducer, initialState) {
  let state = initialState;
  const listeners = [];

  const getState = () => state;

  const dispatch = (action) => {
    state = reducer(state, action);
    listeners.forEach(listener => listener());
  };

  const subscribe = (listener) => {
    listeners.push(listener);
    return () => {
      const index = listeners.indexOf(listener);
      listeners.splice(index, 1);
    };
  };

  return { getState, dispatch, subscribe };
}

// Reducer (pure function)
function counterReducer(state = 0, action) {
  switch(action.type) {
    case 'INCREMENT': return state + 1;
    case 'DECREMENT': return state - 1;
    default: return state;
  }
}

// Usage
const store = createStore(counterReducer);
store.subscribe(() => console.log(store.getState()));
store.dispatch({ type: 'INCREMENT' }); // Logs: 1

---

## 🔹 Best Practices
1. Strive for purity when possible
2. Limit side effects to controlled areas
3. Use immutable data with libraries like Immer
4. Compose small functions into larger ones
5. Document function signatures clearly

---

### 📌 What's Next?
In Part 9, we'll cover:
➡️ Error Handling Strategies
➡️ Debugging Techniques
➡️ Performance Optimization
➡️ Memory Management

#JavaScript #FunctionalProgramming #CleanCode 🚀

Practice Exercise:
1. Convert an imperative function to pure FP style
2. Implement a pipe() function (left-to-right composition)
3. Create a memoization higher-order function

# 📚 JavaScript Tutorial - Part 9/10: Error Handling & Debugging
#JavaScript #Debugging #ErrorHandling #Performance #BestPractices

Welcome to Part 9 of our JavaScript series! Today we'll master professional error handling, debugging techniques, and performance optimization strategies used by senior developers.

---

## 🔹 Comprehensive Error Handling
### 1. Error Types in JavaScript

try {
  // Potential error code
} catch (error) {
  if (error instanceof TypeError) {
    console.log("Type error occurred");
  } else if (error instanceof ReferenceError) {
    console.log("Undefined variable");
  } else if (error instanceof RangeError) {
    console.log("Value out of range");
  } else {
    console.log("Unknown error:", error.message);
  }
}

### 2. Custom Error Classes

class ApiError extends Error {
  constructor(url, status, message) {
    super(`API call to ${url} failed with ${status}: ${message}`);
    this.name = "ApiError";
    this.status = status;
    this.url = url;
  }
}

// Usage
throw new ApiError("/users", 500, "Internal Server Error");

### 3. Error Boundary Pattern (React-like)

function ErrorBoundary({ children }) {
  const [error, setError] = useState(null);

  try {
    return children;
  } catch (err) {
    setError(err);
    return <FallbackUI error={error} />;
  }
}

// Wrap components
<ErrorBoundary>
  <UnstableComponent />
</ErrorBoundary>

---

## 🔹 Advanced Debugging Techniques
### 1. Console Methods Beyond `log()`

console.table([{id: 1, name: 'Ali'}, {id: 2, name: 'Sarah'}]);

console.group("User Details");
console.log("Name: Ali");
console.log("Age: 25");
console.groupEnd();

console.time("API Call");
await fetchData();
console.timeEnd("API Call"); // Logs execution time

### 2. Debugger Statement & Breakpoints

function complexCalculation() {
  debugger; // Pauses execution here
  // Step through with F10/F11
  const result = /* ... */;
  return result;
}

### 3. Source Maps in Production

// webpack.config.js
module.exports = {
  devtool: 'source-map', // Generates .map files
  // ...
};

---

## 🔹 Performance Optimization
### 1. Benchmarking Tools

// Using performance.now()
const start = performance.now();
expensiveOperation();
const end = performance.now();
console.log(`Operation took ${end - start}ms`);

### 2. Memory Leak Detection
Common leak patterns:

// 1. Accidental globals
function leak() {
  leakedVar = 'This is global!'; // Missing var/let/const
}

// 2. Forgotten timers
const intervalId = setInterval(() => {}, 1000);
// Remember to clearInterval(intervalId)

// 3. Detached DOM references
const elements = [];
function storeElement() {
  const el = document.createElement('div');
  elements.push(el); // Keeps reference after removal
}

### 3. Optimization Techniques

// 1. Debounce rapid events
function debounce(fn, delay) {
  let timeout;
  return (...args) => {
    clearTimeout(timeout);
    timeout = setTimeout(() => fn(...args), delay);
  };
}

// 2. Web Workers for CPU-intensive tasks
const worker = new Worker('task.js');
worker.postMessage(data);
worker.onmessage = (e) => console.log(e.data);

// 3. Virtualize long lists (react-window, etc.)

---

## 🔹 Memory Management
### 1. Garbage Collection Basics

// Circular reference (modern engines handle this)
let obj1 = {};
let obj2 = { ref: obj1 };
obj1.ref = obj2;

// Manual cleanup
let heavyResource = loadResource();
function cleanup() {
  heavyResource = null; // Eligible for GC
}

### 2. WeakMap & WeakSet

const weakMap = new WeakMap();
let domNode = document.getElementById('node');
weakMap.set(domNode, { clicks: 0 });

// When domNode is removed, entry is automatically GC'd

---

## 🔹 Network Optimization
### 1. Bundle Analysis

npm install -g source-map-explorer
source-map-explorer bundle.js

### 2. Code Splitting

// Dynamic imports
const module = await import('./heavyModule.js');

// React.lazy
const HeavyComponent = React.lazy(() => import('./HeavyComponent'));

### 3. Caching Strategies

// Service Worker caching
self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.match(event.request)
      .then(response => response || fetch(event.request))
  );
});

---

## 🔹 Security Best Practices
### 1. Input Sanitization

function sanitizeInput(input) {
  return input.replace(/</g, '&lt;').replace(/>/g, '&gt;');
}

### 2. Content Security Policy (CSP)

<meta http-equiv="Content-Security-Policy" 
      content="default-src 'self'; script-src 'self' 'unsafe-inline'">

### 3. Secure Authentication

// Never store tokens in localStorage
const auth = {
  getToken() {
    return window.sessionStorage.getItem('token');
  },
  setToken(token) {
    window.sessionStorage.setItem('token', token);
  }
};

---

## 🔹 Practical Example: Robust API Wrapper

class ApiClient {
  constructor(baseUrl) {
    this.baseUrl = baseUrl;
    this.cache = new Map();
  }

  async request(endpoint, options = {}) {
    const cacheKey = JSON.stringify({ endpoint, options });
    
    try {
      // Cache-first strategy
      if (this.cache.has(cacheKey)) {
        return this.cache.get(cacheKey);
      }

      const response = await fetch(`${this.baseUrl}${endpoint}`, {
        headers: { 'Content-Type': 'application/json' },
        ...options
      });

      if (!response.ok) {
        throw new ApiError(
          endpoint, 
          response.status, 
          await response.text()
        );
      }

      const data = await response.json();
      this.cache.set(cacheKey, data); // Cache response
      return data;

    } catch (error) {
      if (error instanceof ApiError) {
        console.error(`API Error: ${error.message}`);
      } else {
        console.error(`Network Error: ${error.message}`);
      }
      throw error;
    }
  }
}

---

## 🔹 Best Practices Checklist
1. Error Handling
- [ ] Use specific error types
- [ ] Implement global error handlers
- [ ] Validate API responses

2. Debugging
- [ ] Utilize source maps
- [ ] Leverage browser dev tools
- [ ] Add strategic debugger statements

3. Performance
- [ ] Audit bundle size regularly
- [ ] Implement lazy loading
- [ ] Debounce rapid events

4. Security
- [ ] Sanitize user input
- [ ] Use secure token storage
- [ ] Implement CSP headers

---

### 📌 What's Next?
In Final Part 10, we'll cover:
➡️ Modern JavaScript (ES2023+)
➡️ TypeScript Fundamentals
➡️ Advanced Patterns
➡️ Where to Go From Here

#JavaScript #ProfessionalDevelopment #WebDev 🚀

Practice Exercise:
1. Implement error boundaries in a React/Vue app
2. Profile a slow function using Chrome DevTools
3. Create a memory leak and detect it

# 📚 JavaScript Tutorial - Part 10/10: Modern JavaScript & Beyond
#JavaScript #ES2023 #TypeScript #AdvancedPatterns #WebDev

Welcome to the final part of our comprehensive JavaScript series! This ultimate lesson explores cutting-edge JavaScript features, TypeScript fundamentals, advanced patterns, and pathways for continued learning.

---

## 🔹 Modern JavaScript Features (ES2023+)
### 1. Top-Level Await (ES2022)

// Module.js
const data = await fetch('https://api.example.com/data');
export { data };

// No need for async wrapper!

### 2. Array Find from Last (ES2023)

const numbers = [10, 20, 30, 20, 40];
numbers.findLast(n => n === 20); // 20 (last occurrence)
numbers.findLastIndex(n => n === 20); // 3

### 3. Hashbang Support (ES2023)

#!/usr/bin/env node
// Now executable directly via ./script.js
console.log('Hello from executable JS!');

### 4. WeakRef & FinalizationRegistry (ES2021)

const weakRef = new WeakRef(domElement);
const registry = new FinalizationRegistry(heldValue => {
  console.log(`${heldValue} was garbage collected`);
});

registry.register(domElement, "DOM Element");

### 5. Error Cause (ES2022)

try {
  await fetchData();
} catch (error) {
  throw new Error('Processing failed', { cause: error });
}

---

## 🔹 TypeScript Fundamentals
### 1. Basic Types

let username: string = "Ali";
let age: number = 25;
let isActive: boolean = true;
let scores: number[] = [90, 85, 95];
let user: { name: string; age?: number } = { name: "Ali" };

### 2. Interfaces & Types

interface User {
  id: number;
  name: string;
  email: string;
}

type Admin = User & { 
  permissions: string[];
};

function createUser(user: User): Admin {
  // ...
}

### 3. Generics

function identity<T>(arg: T): T {
  return arg;
}

const result = identity<string>("Hello");

### 4. Type Inference & Utility Types

const user = {
  name: "Ali",
  age: 25
}; // Type inferred as { name: string; age: number }

type PartialUser = Partial<typeof user>;
type ReadonlyUser = Readonly<typeof user>;

---

## 🔹 Advanced Patterns
### 1. Dependency Injection

class Database {
  constructor(private connection: Connection) {}
  
  query(sql: string) {
    return this.connection.execute(sql);
  }
}

const db = new Database(new MySQLConnection());

### 2. Proxy API

const validator = {
  set(target, property, value) {
    if (property === 'age' && typeof value !== 'number') {
      throw new TypeError('Age must be a number');
    }
    target[property] = value;
    return true;
  }
};

const user = new Proxy({}, validator);
user.age = 25; // OK
user.age = "25"; // Throws error

### 3. Observable Pattern

class Observable<T> {
  private subscribers: ((value: T) => void)[] = [];

  subscribe(callback: (value: T) => void) {
    this.subscribers.push(callback);
  }

  next(value: T) {
    this.subscribers.forEach(cb => cb(value));
  }
}

const clicks = new Observable<MouseEvent>();
clicks.subscribe(e => console.log(e.clientX));
document.addEventListener('click', e => clicks.next(e));

---

## 🔹 Web Components
### 1. Custom Elements

class PopupAlert extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: 'open' });
    this.shadowRoot.innerHTML = `
      <style>
        .alert { /* styles */ }
      </style>
      <div class="alert">
        <slot></slot>
      </div>
    `;
  }
}

customElements.define('popup-alert', PopupAlert);

### 2. Template Literals for HTML

function html(strings, ...values) {
  let str = '';
  strings.forEach((string, i) => {
    str += string + (values[i] || '');
  });
  const template = document.createElement('template');
  template.innerHTML = str;
  return template.content;
}

const fragment = html`<div>Hello ${name}</div>`;

---

## 🔹 Performance Patterns
### 1. Virtualization

function renderVirtualList(items, container, renderItem) {
  const visibleItems = getVisibleItems(items, container);
  container.replaceChildren(
    ...visibleItems.map(item => renderItem(item))
  );
}