JavaScript

JavaScript is the programming language of the web. It enables interactive user interfaces, asynchronous operations, server-side applications, desktop applications, and more.

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Learning Path

Fundamentals

• Variables (var, let, const)
• Data Types
• Type Coercion
• Operators
• Scope
• Hoisting
• Functions
• Closures
• Objects
• Arrays

Intermediate

• Event Loop
• Call Stack
• Execution Context
• Prototype Chain
• Classes
• Modules
• Error Handling
• Promises
• Async/Await

Advanced

• Memory Management
• Garbage Collection
• Event Delegation
• Design Patterns
• Performance Optimization
• Functional Programming
• Web Workers

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Example: Closures

A closure allows a function to access variables from its outer scope even after the outer function has returned.

function counter() {
  let count = 0;

  return function () {
    count++;
    return count;
  };
}

const increment = counter();

console.log(increment()); // 1
console.log(increment()); // 2
console.log(increment()); // 3

Why it Works

1. counter() creates a local variable count.
2. The returned function references count.
3. JavaScript preserves that variable in memory.
4. Each call updates the same variable.

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Common Interview Question

What is the difference between let, const and var?

**var**

- Function scoped
- Can be redeclared
- Hoisted and initialized with `undefined`

**let**

- Block scoped
- Cannot be redeclared in same scope
- Exists in Temporal Dead Zone

**const**

- Block scoped
- Cannot be reassigned
- Must be initialized during declaration

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Important Notes

Note

JavaScript is single-threaded but can perform asynchronous operations through the Event Loop.

Warning

Avoid using `var` in modern applications unless maintaining legacy code.

Tip

Master closures, promises, async/await, event loop, and prototype inheritance for senior-level interviews.

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Event Loop Overview

Call Stack
    |
    v
Web APIs
    |
    v
Callback Queue
    |
    v
Event Loop
    |
    v
Call Stack

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Frequently Asked Topics

Topic	SDE-1	SDE-2
Scope	✅	✅
Closures	✅	✅
Event Loop	✅	✅
Prototypes	⚠️	✅
Memory Management	❌	✅
Design Patterns	❌	✅

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Recommended Study Order

1. Variables and Scope
2. Functions
3. Closures
4. Objects
5. Arrays
6. Execution Context
7. Event Loop
8. Promises
9. Async/Await
10. Prototype Chain
11. Memory Management
12. Design Patterns

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Next Steps

Continue with:

• Fundamentals → Scope & Hoisting
• Fundamentals → Functions
• Fundamentals → Closures
• Browser → Event Loop
• Advanced → Memory Management