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CS Notes

Overview

What is JavaScript?

Section titled “What is JavaScript?”

JavaScript is a high-level, interpreted, dynamically typed programming language primarily used to create interactive and dynamic web applications. It runs inside the browser but is also widely used on servers through environments like Node.js.

Unlike traditional compiled languages, JavaScript executes directly in the browser using a JavaScript Engine (like V8 in Chrome), which parses and executes code line by line.

How JavaScript Works Internally

Section titled “How JavaScript Works Internally”

JavaScript follows a single-threaded, event-driven architecture using an event loop to handle asynchronous operations.

graph TD A[Call Stack] -->|Executes| B[Function Execution] C[Web APIs] --> D[Callback Queue] D -->|Event Loop| A A --> E[Output] subgraph Async Flow C D end

Explanation

Section titled “Explanation”
  • Call Stack → Executes functions (LIFO structure)
  • Web APIs → Browser-provided APIs (setTimeout, fetch, DOM)
  • Callback Queue → Stores async callbacks
  • Event Loop → Moves tasks from queue to stack when it’s empty

Core Features of JavaScript

Section titled “Core Features of JavaScript”

Dynamic Typing

Variables do not require explicit types and can change type at runtime.

First-Class Functions

Functions can be assigned to variables, passed as arguments, and returned from other functions.

Prototype-Based OOP

Objects inherit from other objects using prototypes instead of classical classes.

Event-Driven

JavaScript responds to user actions like clicks, typing, and network responses.

JavaScript Execution Flow

Section titled “JavaScript Execution Flow”
graph LR A[Write Code] --> B[JS Engine] B --> C[Parsing] C --> D[Compilation] D --> E[Execution]

Breakdown

Section titled “Breakdown”
  1. Parsing → Converts code into Abstract Syntax Tree (AST)
  2. Compilation → Optimized into bytecode or machine code
  3. Execution → Runs inside the JavaScript engine

JavaScript Ecosystem Overview

Section titled “JavaScript Ecosystem Overview”

Frontend

Used in browsers to manipulate UI using DOM, events, and APIs.

Backend (Node.js)

Server-side JavaScript for APIs, databases, and services.

Frameworks & Libraries

React, Vue, Angular help build scalable applications.

Tooling

Bundlers (Webpack, Vite), Transpilers (Babel), Linters (ESLint)

Topics Covered in This Section

Section titled “Topics Covered in This Section”

  1. Variables & Data Types Understanding var, let, const, primitive and reference types.

  2. Operators & Expressions Arithmetic, logical, comparison, and assignment operations.

  3. Control Flow Conditional statements (if, switch) and loops (for, while).

  4. Functions Function declarations, expressions, arrow functions, closures.

  5. Objects & Arrays Core data structures used in JavaScript.

  6. DOM Manipulation Interacting with HTML elements dynamically.

  7. Events Handling user interactions like clicks, input, and keyboard events.

  8. Asynchronous JavaScript Callbacks, Promises, async/await.

  9. Prototypes & Object-Oriented Programming Understanding prototypes, inheritance, and OOP concepts.

  10. ES6+ Features Modern syntax like destructuring, spread operator, modules.

  11. Unexpected JavaScript Behavior Deep dive into surprising language behavior and how to avoid production bugs.

Synchronous vs Asynchronous Execution

Section titled “Synchronous vs Asynchronous Execution”

Code runs line by line, blocking execution.

graph TD A[Task 1] --> B[Task 2] B --> C[Task 3]

JavaScript Runtime Environment

Section titled “JavaScript Runtime Environment”
graph TD A[JavaScript Code] --> B[Engine] B --> C[Memory Heap] B --> D[Call Stack]

Explanation

Section titled “Explanation”
  • Memory Heap → Stores objects and variables
  • Call Stack → Keeps track of function execution

Why Learn JavaScript?

Section titled “Why Learn JavaScript?”
  • Essential for web development
  • Works on frontend + backend
  • Huge ecosystem and community
  • High demand in industry