We’ll cover concept → setup → routing → middleware → REST API → MongoDB connection → error handling → deployment basics — all with code blocks and clear explanation.

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⚙️ 1. What is Express.js?

Express.js is a minimal and flexible Node.js web application framework
used to build APIs, backend logic, and web servers easily.

🧩 Role in MERN:
It acts as the backend server — connects the frontend (React) with database (MongoDB).

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🧱 2. Setup Project

Step 1 — Initialize Node.js

mkdir express-demo
cd express-demo
npm init -y

Step 2 — Install Express

npm install express

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🚀 3. Basic Server Setup

📁 index.js

// Import express
const express = require("express");

// Initialize app
const app = express();

// Port number
const PORT = 5000;

// Middleware to parse JSON
app.use(express.json());

// Basic route
app.get("/", (req, res) => {
  res.send("Hello from Express Server 🚀");
});

// Start server
app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

👉 Run it:

node index.js

Visit http://localhost:5000

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🛣️ 4. Routing Basics

📁 index.js

app.get("/users", (req, res) => {
  res.send("Get all users");
});

app.post("/users", (req, res) => {
  res.send("Create new user");
});

app.put("/users/:id", (req, res) => {
  res.send(`Update user ${req.params.id}`);
});

app.delete("/users/:id", (req, res) => {
  res.send(`Delete user ${req.params.id}`);
});

✅ You just built a basic REST API.

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🧩 5. Middleware

Middlewares are functions that process requests before reaching routes.

// Custom middleware example
const logger = (req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // pass control to next handler
};

app.use(logger);

Now every request will be logged.

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🧠 6. Request and Response

app.post("/data", (req, res) => {
  const { name, age } = req.body;
  res.json({ message: `Received ${name}, age ${age}` });
});

➡️ Send JSON using Postman:

{
  "name": "Yuvaraj",
  "age": 22
}

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🧱 7. Express Router (Modular Routes)

📁 routes/userRoutes.js

const express = require("express");
const router = express.Router();

router.get("/", (req, res) => res.send("All users"));
router.post("/", (req, res) => res.send("User added"));

module.exports = router;

📁 index.js

const userRoutes = require("./routes/userRoutes");
app.use("/api/users", userRoutes);

✅ Now visit → /api/users

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🍃 8. Connect to MongoDB (with Mongoose)

Install:

npm install mongoose

📁 db.js

const mongoose = require("mongoose");

const connectDB = async () => {
  try {
    await mongoose.connect("mongodb://localhost:27017/expressdb");
    console.log("✅ MongoDB Connected");
  } catch (err) {
    console.error("❌ DB Connection Failed:", err.message);
  }
};

module.exports = connectDB;

In index.js

const connectDB = require("./db");
connectDB();

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🧩 9. Model and CRUD Example

📁 models/User.js

const mongoose = require("mongoose");

const userSchema = new mongoose.Schema({
  name: String,
  email: String,
  age: Number
});

module.exports = mongoose.model("User", userSchema);

📁 routes/userRoutes.js

const express = require("express");
const router = express.Router();
const User = require("../models/User");

// Create User
router.post("/", async (req, res) => {
  const user = new User(req.body);
  await user.save();
  res.json(user);
});

// Get All Users
router.get("/", async (req, res) => {
  const users = await User.find();
  res.json(users);
});

// Get by ID
router.get("/:id", async (req, res) => {
  const user = await User.findById(req.params.id);
  res.json(user);
});

// Update
router.put("/:id", async (req, res) => {
  const user = await User.findByIdAndUpdate(req.params.id, req.body, { new: true });
  res.json(user);
});

// Delete
router.delete("/:id", async (req, res) => {
  await User.findByIdAndDelete(req.params.id);
  res.json({ message: "User deleted" });
});

module.exports = router;

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🧰 10. Error Handling Middleware

// Error handling
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ message: "Something broke!" });
});

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🧠 11. Environment Variables (dotenv)

Install:

npm install dotenv

📁 .env

PORT=5000
MONGO_URI=mongodb://localhost:27017/expressdb

Update index.js

require("dotenv").config();
const PORT = process.env.PORT || 3000;

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🔒 12. CORS (Frontend Connection)

Install:

npm install cors

In index.js

const cors = require("cors");
app.use(cors());

Now your React app can make API calls safely.

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🚀 13. Folder Structure

express-demo/
│
├── models/
│   └── User.js
│
├── routes/
│   └── userRoutes.js
│
├── db.js
├── index.js
├── .env
└── package.json

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🌐 14. Deployment Basics

For production:

npm install --save-dev nodemon

In package.json:

"scripts": {
  "start": "node index.js",
  "dev": "nodemon index.js"
}

Then:

npm run dev

✅ Deploy on:

• Render, Vercel, or Railway for backend hosting.

• MongoDB Atlas for online DB.

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✅ Summary

Component	Role
express	Framework to handle routes, requests, middleware
mongoose	ODM for MongoDB
dotenv	Manage secrets
cors	Connect with frontend safely
nodemon	Auto restart server during development
Routes	Define API endpoints
Models	Define DB schema

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