1. Overview of MERN Stack:

• MongoDB: NoSQL database for storing data in JSON-like documents.
• Express.js: Web application framework for Node.js to build APIs.
• React.js: Front-end library for building user interfaces.
• Node.js: JavaScript runtime for executing server-side code.

2. MongoDB:

• Basic Commands:

db.collection.find(): Query documents.
db.collection.insertOne(): Insert a single document.
db.collection.updateOne(): Update a document.
db.collection.deleteOne(): Delete a document.

• Mongoose:
  • Use Mongoose for schema definitions and validation.
  • Example Schema:
    
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    const mongoose = require('mongoose');
    const UserSchema = new mongoose.Schema({
        name: String,
        email: String,
        password: String,
    });
    
    

3. Express.js:

• Basic Setup:
  
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  const express = require('express');
  const app = express();
  app.use(express.json());
  
  

• Routing:
  • Create routes using app.get(), app.post(), etc.
  • Use express.Router() for modular routing.
• Middleware:
  • Use middleware for error handling, authentication, and logging.

4. React.js:

• Component Structure:
  • Use functional components and hooks (e.g., useState, useEffect).
  • Example Component:
    
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    import React, { useState } from 'react';
    
    const Counter = () => {
        const [count, setCount] = useState(0);
        return <button onClick={() => setCount(count + 1)}>{count}</button>;
    };
    
    

• State Management:
  • Use Context API or libraries like Redux for global state management.
• Routing:
  • Use react-router-dom for navigation between components.
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  import { BrowserRouter as Router, Route } from 'react-router-dom';
  

5. Node.js:

• Basic Server Setup:
  
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  const http = require('http');
  const server = http.createServer(app);
  server.listen(3000, () => {
      console.log('Server is running on port 3000');
  });
  
  

• APIs:
  • Build RESTful APIs and connect them to the front-end.

6. Full-Stack Integration:

• API Calls: Use Axios or Fetch API in React to make HTTP requests to the Express backend.
• CORS: Enable CORS in Express for cross-origin requests.
  
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  const cors = require('cors');
  app.use(cors());
  
  

7. Authentication:

• JWT (JSON Web Tokens):
  • Use JWT for user authentication.
  • Example:
    
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    const jwt = require('jsonwebtoken');
    const token = jwt.sign({ userId: user._id }, 'secretkey');
    
    

• Passport.js: Optional library for more complex authentication strategies.

This Elysium Spark Note provides a solid foundation for mastering the MERN stack. Focus on building projects to apply these concepts, and keep exploring advanced topics as you MERN stack progress!