backend

1. Node.js Basics

What is Node.js?

• Node.js is a runtime environment that allows JavaScript to run outside the browser.

• Built on Chrome's V8 engine.

• Used for backend development, real-time applications, APIs, etc.

Key Features

• Non-blocking, event-driven architecture.

• Uses single-threaded event loop.

• Fast and scalable.

2. Modules in Node.js

Types of Modules

• Built-in modules: fs, http, path, url, crypto, etc.

• Custom modules: Your own files (e.g., require('./myModule')).

• Third-party modules: Installed using NPM (e.g., express, mongoose, etc.).

Module Systems

• CommonJS (CJS)

  • Used by default in Node.js.

  • require() and module.exports

• ES Modules (ESM)

  • Modern JavaScript module system.

  • Use import and export

  • Requires "type": "module" in package.json

3. NPM (Node Package Manager)

Commands

• npm init / npm init -y: Initialize project.

• npm install <package>: Install a package.

• npm install --save-dev <package>: Dev dependency.

• npm uninstall <package>: Remove a package.

package.json file manages dependencies and project metadata.

4. Express.js

What is Express?

• A minimal and flexible Node.js web application framework.

• Simplifies routing, middleware, request/response handling.

Basic Setup

const express = require('express');
const app = express();
const PORT = 3000;

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

5. Basic API Structure (CRUD)

const express = require('express');
const app = express();

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

// CREATE
app.post('/items', (req, res) => {
  const item = req.body;
  // logic to save item
  res.status(201).send('Item created');
});

// READ
app.get('/items', (req, res) => {
  res.send(['item1', 'item2']);
});

// UPDATE
app.put('/items/:id', (req, res) => {
  const id = req.params.id;
  res.send(`Item ${id} updated`);
});

// DELETE
app.delete('/items/:id', (req, res) => {
  const id = req.params.id;
  res.send(`Item ${id} deleted`);
});

app.listen(3000);

6. Middleware in Express

What is Middleware?

• Functions that run between request and response.

• Can modify req and res objects.

• Can end the request-response cycle or pass to next middleware.

Types

• Application-level middleware: app.use() or route-specific.

• Router-level middleware: router.use()

• Built-in middleware: express.json(), express.static()

• Error-handling middleware: Takes 4 args (err, req, res, next)

Example

app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next();
});

7. Routing in Express

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

app.get('/about', (req, res) => {
  res.send('About Page');
});

You can also use express.Router() for modular routes.

8. Error Handling

Simple error handler middleware

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

9. Environment Variables

Use .env file

Install dotenv

npm install dotenv

In your entry file:

require('dotenv').config();
const PORT = process.env.PORT;

10. Project Structure (Basic)

project/
|-- node_modules/
|-- routes/
|   |-- itemRoutes.js
|-- controllers/
|   |-- itemController.js
|-- models/
|   |-- itemModel.js
|-- .env
|-- index.js
|-- package.json

This structure is clean and modular, good for scaling.

11. Important Middlewares/Libraries to Know

• body-parser (mostly replaced by express.json())

• cors – handle cross-origin requests

• helmet – security headers

• morgan – logging HTTP requests

• express-validator – input validation

• jsonwebtoken – JWT token handling for auth

12. Extra Concepts to Revise

• RESTful API design

• HTTP methods: GET, POST, PUT, PATCH, DELETE

• HTTP status codes: 200, 201, 400, 401, 403, 404, 500

• Authentication: Sessions vs JWT

• Rate limiting and basic security

• Asynchronous operations (async/await, Promises)

• Connecting with MongoDB or SQL

End of One-Shot Notes.