#Paper
What Every Programmer Should Know About Memory
You must read this paper.

Git Basics

SOAP vs REST vs GraphQL vs RPC

Visualizing SQL Queries

Note: The actual execution sequence may differ from this mental model due to optimization strategies employed by the query optimizer.

SQL Question:

What does this query return for a NULL score?

What is API Gateway?

✅ 50 Must-Know Web Development Concepts for Interviews 🌐💼

📍 HTML Basics 
1. What is HTML? 
2. Semantic tags (article, section, nav) 
3. Forms and input types 
4. HTML5 features 
5. SEO-friendly structure 

📍 CSS Fundamentals 
6. CSS selectors & specificity 
7. Box model 
8. Flexbox 
9. Grid layout 
10. Media queries for responsive design 

📍 JavaScript Essentials
11. let vs const vs var 
12. Data types & type coercion 
13. DOM Manipulation 
14. Event handling 
15. Arrow functions 

📍 Advanced JavaScript 
16. Closures 
17. Hoisting 
18. Callbacks vs Promises 
19. async/await 
20. ES6+ features 

📍 Frontend Frameworks
21. React: props, state, hooks 
22. Vue: directives, computed properties 
23. Angular: components, services 
24. Component lifecycle 
25. Conditional rendering 

📍 Backend Basics
26. Node.js fundamentals 
27. Express.js routing 
28. Middleware functions 
29. REST API creation 
30. Error handling 

📍 Databases 
31. SQL vs NoSQL 
32. MongoDB basics 
33. CRUD operations 
34. Indexes & performance 
35. Data relationships 

📍 Authentication & Security 
36. Cookies vs LocalStorage 
37. JWT (JSON Web Token) 
38. HTTPS & SSL 
39. CORS 
40. XSS & CSRF protection 

📍 APIs & Web Services
41. REST vs GraphQL 
42. Fetch API 
43. Axios basics 
44. Status codes 
45. JSON handling 

📍 DevOps & Tools 
46. Git basics & GitHub 
47. CI/CD pipelines 
48. Docker (basics) 
49. Deployment (Netlify, Vercel, Heroku) 
50. Environment variables (.env)

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✅ HTML Basics – Interview Questions & Answers 📄

1️⃣ What is HTML?
Answer: HTML (HyperText Markup Language) is the standard language used to structure content on the web. It defines elements like headings, paragraphs, links, images, and forms using tags.

2️⃣ What are semantic tags in HTML?
Answer: Semantic tags clearly describe their meaning in the context of the page. Examples:
- <article> – for self-contained content
- <section> – for grouped content
- <nav> – for navigation links
They improve accessibility and SEO.

3️⃣ What are forms and input types in HTML?
Answer: Forms collect user input. Common input types include:
- text, email, password, checkbox, radio, submit
Example:

<form>
  <input type="email" placeholder="Enter your email" />
</form>

4️⃣ What are key features of HTML5?
Answer:
- New semantic tags (<header>, <footer>, <main>)
- Native audio/video support (<audio>, <video>)
- Local storage & session storage
- Canvas for graphics
- Geolocation API

5️⃣ How do you create an SEO-friendly HTML structure?
Answer:
- Use semantic tags
- Include proper heading hierarchy (<h1> to <h6>)
- Add alt attributes to images
- Use denoscriptive noscripts and meta tags
- Ensure fast loading and mobile responsiveness

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✅ CSS Fundamentals – Interview Questions & Answers 🎨🧠

1️⃣ What is the Box Model in CSS?
The box model describes how elements are rendered: 
Content -> Padding -> Border -> Margin
It affects spacing and layout.

2️⃣ What’s the difference between ID and Class selectors?
- #id: Unique, used once. 
- .class: Reusable across multiple elements. 
Example: 

#header { color: red; }  
.card { padding: 10px; }

3️⃣ How does CSS Specificity work? 
Specificity decides which styles are applied when multiple rules target the same element. 
Hierarchy: 
Inline > ID > Class > Element 
Example: 

<p id="one" class="two">Text</p>

#one has higher specificity than .two.

4️⃣ What is Flexbox?
A layout model for 1D alignment (row or column). 
Key properties: 
- display: flex
- justify-content, align-items,
flex-wrap

5️⃣ Difference between Flexbox and Grid?
- Flexbox: 1D layout (row/column). 
- Grid: 2D layout (rows & columns). 
Use Grid when layout needs both directions.

6️⃣ What are Media Queries? 
Used to create responsive designs based on screen size/device. 
Example: 

@media (max-width: 600px) {
  body { font-size: 14px; }
}

7️⃣ How do you center a div using Flexbox? 

.container {
  display: flex;
  justify-content: center;
  align-items: center;
}

8️⃣ What is the difference between position: relative and absolute?
- relative: positions relative to itself. 
- absolute: positions relative to the nearest positioned ancestor.

9️⃣ Explain z-index in CSS. 
Controls stacking order of elements. Higher z-index = appears on top.

🔟 How can you optimize CSS performance? 
- Minify files 
- Use shorthand properties 
- Combine selectors 
- Avoid deep nesting 
- Use external stylesheets

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✅ JavaScript Essentials – Interview Questions with Answers🧠💻 

1️⃣ Q: What is the difference between let, const, and var?
A: 
- var: Function-scoped, hoisted, can be redeclared. 
- let: Block-scoped, not hoisted like var, can't be redeclared in same scope. 
- const: Block-scoped, must be assigned at declaration, cannot be reassigned.

2️⃣ Q: What are JavaScript data types? 
A:
- Primitive types: string, number, boolean, null, undefined, symbol, bigint 
- Non-primitive: object, array, function 
Type coercion: JS automatically converts between types in operations ('5' + 2 → '52')

3️⃣ Q: How does DOM Manipulation work in JS?
A:
The DOM (Document Object Model) represents the HTML structure. JS can access and change elements using: 
- document.getElementById() 
- document.querySelector() 
- element.innerText, element.innerHTML, element.style etc.

4️⃣ Q: What is event handling in JavaScript?
A: 
It allows reacting to user actions like clicks or key presses. 
Example: 

document.getElementById("btn").addEventListener("click", () => {
  alert("Button clicked!");
});

5️⃣ Q: What are arrow functions?
A:
A shorter syntax for functions introduced in ES6. 

const add = (a, b) => a + b;

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✅ Advanced JavaScript Interview Questions with Answers 💼🧠

1. What is a closure in JavaScript?
A *closure* is a function that has access to its outer function's variables, even after the outer function has returned.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    console.log(count);
  }
}
const counter = outer();
counter(); // 1
counter(); // 2

2. Explain event delegation.
Event delegation is a technique where you attach a single event listener to a parent element to handle events on its child elements using event.target.

3. What is the difference between == and ===? 
- == checks for value equality (type coercion allowed). 
- === checks for both value and type (strict equality). 

'5' == 5   // true  
'5' === 5  // false

4. What is the "this" keyword?
this refers to the object that is executing the current function. In arrow functions, this is lexically bound (based on where it's defined).

5. What are Promises? 
Promises handle asynchronous operations. They have 3 states: *pending, resolved, rejected*.

const p = new Promise((resolve, reject) => {
  resolve("Success");
});
p.then(console.log);

6. Explain async/await.
Syntactic sugar over Promises for better readability in async code.

async function fetchData() {
  const res = await fetch(url);
  const data = await res.json();
}

7. What is hoisting?
Variables and function declarations are moved to the top of their scope before code execution.

console.log(a); // undefined
var a = 5;

8. What are arrow functions and how do they differ? 
Arrow functions are shorter and do not have their own this, arguments, or super.

const add = (a, b) => a + b;

9. What is the event loop?
The event loop handles asynchronous callbacks and ensures non-blocking behavior in JS by placing them in the task queue after the call stack is clear.

10. What are IIFEs (Immediately Invoked Function Expressions)?
Functions that run as soon as they are defined.

(function() {
  console.log("Runs immediately");
})();

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✅  Frontend Frameworks Interview Q&A – Part 1 🌐💼

1️⃣ What are props in React?
Answer: Props (short for properties) are used to pass data from parent to child components. They are read-only and help make components reusable.

2️⃣ What is state in React?
Answer: State is a built-in object used to store dynamic data that affects how the component renders. Unlike props, state can be changed within the component.

3️⃣ What are React hooks?
*Answer:* Hooks like useState, useEffect, and useContext let you use state and lifecycle features in functional components without writing class components.

4️⃣ What are directives in Vue.js?
Answer: Directives are special tokens in Vue templates that apply reactive behavior to the DOM. Examples include v-if, v-for, and v-bind.

5️⃣ What are computed properties in Vue?
Answer: Computed properties are cached based on their dependencies and only re-evaluate when those dependencies change — great for performance and cleaner templates.

6️⃣ What is a component in Angular?
Answer: A component is the basic building block of Angular apps. It includes a template, class, and metadata that define its behavior and appearance.

7️⃣ What are services in Angular?
Answer: Services are used to share data and logic across components. They’re typically injected using Angular’s dependency injection system.

8️⃣ What is conditional rendering?
Answer: Conditional rendering means showing or hiding UI elements based on conditions. In React, you can use ternary operators or logical && to do this.

9️⃣ What is the component lifecycle in React?
*Answer:* Lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount manage side effects and updates in class components. In functional components, use useEffect.

🔟 How do frameworks improve frontend development?
*Answer:* They offer structure, reusable components, state management, and better performance — making development faster, scalable, and more maintainable.

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✅ Frontend Frameworks Interview Q&A – Part 2 🌐💼 

1️⃣ What is Virtual DOM in React?
Answer:
The Virtual DOM is a lightweight copy of the real DOM. React updates it first, calculates the difference (diffing), and then efficiently updates only what changed in the actual DOM.

2️⃣ Explain data binding in Angular.
Answer: 
Angular supports one-way, two-way ([(ngModel)]), and event binding to sync data between the component and the view.

3️⃣ What is JSX in React?
Answer:
JSX stands for JavaScript XML. It allows you to write HTML-like syntax inside JavaScript, which is compiled to React’s createElement() calls.

4️⃣ What are slots in Vue.js?
Answer: 
Slots allow you to pass template content from parent to child components, making components more flexible and reusable.

5️⃣ What is lazy loading in Angular or React? 
Answer: 
Lazy loading is a performance optimization technique that loads components or modules only when needed, reducing initial load time.

6️⃣ What are fragments in React?
Answer:
<React.Fragment> or <> lets you group multiple elements without adding extra nodes to the DOM.

7️⃣ How do you lift state up in React? 
Answer:
By moving the shared state to the closest common ancestor of the components that need it, and passing it down via props.

8️⃣ What is a watch property in Vue?
Answer:
watch allows you to perform actions when data changes — useful for async operations or side effects.

9️⃣ What is dependency injection in Angular?
Answer:
A design pattern where Angular provides objects (like services) to components, reducing tight coupling and improving testability.

🔟 What is server-side rendering (SSR)?
Answer:
SSR renders pages on the server, not the browser. It improves SEO and load times. Examples: Next.js (React), Nuxt.js (Vue), Angular Universal.

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✅ Backend Basics Interview Questions – Part 1 (Node.js)🧠💻

📍 1. What is Node.js?
A: Node.js is a runtime environment that lets you run JavaScript on the server side. It uses Google’s V8 engine and is designed for building scalable network applications.

📍 2. How is Node.js different from traditional server-side platforms?
A: Unlike PHP or Java, Node.js is event-driven and non-blocking. This makes it lightweight and efficient for I/O-heavy operations like APIs and real-time apps.

📍 3. What is the role of the package.json file?
A: It stores metadata about your project (name, version, noscripts) and dependencies. It’s essential for managing and sharing Node.js projects.

📍 4. What are CommonJS modules in Node.js? 
A: Node uses CommonJS to handle modules. You use require() to import and module.exports to export code between files.

📍 5. What is the Event Loop in Node.js?
A: It allows Node.js to handle many connections asynchronously without blocking. It’s the heart of Node’s non-blocking architecture.

📍 6. What is middleware in Node.js (Express)? 
A: Middleware functions process requests before sending a response. They can be used for logging, auth, validation, etc.

📍 7. What is the difference between process.nextTick(), setTimeout(), and setImmediate()? 
A: 
- process.nextTick() runs after the current operation, before the next event loop. 
- setTimeout() runs after a minimum delay. 
- setImmediate() runs on the next cycle of the event loop.

📍 8. What is a callback function in Node.js?
A: A function passed as an argument to another function, executed after an async task finishes. It’s the core of async programming in Node.

📍 9. What are Streams in Node.js?
A: Streams let you read/write data piece-by-piece (chunks), great for handling large files. Types: Readable, Writable, Duplex, Transform.

📍 10. What is the difference between require and import?
A:
- require is CommonJS (used in Node.js by default). 
- import is ES6 module syntax (used with "type": "module" in package.json).

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✅ Backend Basics Interview Questions – Part 2 (Express.js Routing) 🚀🧠

📍 1. What is Routing in Express.js?
A: Routing defines how your application responds to client requests (GET, POST, etc.) to specific endpoints (URLs).

📍 2. Basic Route Syntax

app.get('/home', (req, res) => {
  res.send('Welcome Home');
});

📍3. Route Methods 
- app.get() – Read data 
- app.post() – Create data 
- app.put() – Update data 
- app.delete() – Delete data 

📍 4. Route Parameters

app.get('/user/:id', (req, res) => {
  res.send(req.params.id);
});

📍 5. Query Parameters 

app.get('/search', (req, res) => {
  res.send(req.query.keyword);
});

📍 6. Route Chaining

app.route('/product')
  .get(getHandler)
  .post(postHandler)
  .put(putHandler);

📍 7. Router Middleware

const router = express.Router();
router.get('/about', (req, res) => res.send('About Page'));
app.use('/info', router);  // URL: /info/about

📍 8. Error Handling Route

app.use((req, res) => {
  res.status(404).send('Page Not Found');
});

💡 Pro Tip: Always place dynamic routes after static ones to avoid conflicts.

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