Using Express

Express is a popular Node.js framework that simplifies the development of web applications and APIs by providing a robust set of features and tools. In this section, we'll cover the basics of setting up an Express application, defining routes, using middleware, and connecting to a database.

Setting Up Express

First, let's set up a basic Express application. Ensure you have Node.js installed, then follow these steps:

Step 1: Initialize a New Node.js Project

commands.sh

mkdir my-express-app
cd my-express-app
npm init -y

Step 2: Install Express

commands.sh

npm install express

Step 3: Create a Basic Server

Create a file named app.js and add the following code to set up a basic Express server.

app.js

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

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

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

Defining Routes

Routes are used to define how an application responds to client requests for a specific endpoint. Let's define some basic routes.

Basic Routing

basic-routing.js

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

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

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

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

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

Route Parameters

Route parameters are named URL segments used to capture values at specific positions in the URL.

route-params.js

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

app.get('/users/:userId', (req, res) => {
  res.send(`User ID: ${req.params.userId}`);
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

                        localhost:3000/users/{IdNum}
Example running on http://localhost:3000/users/123

Query Parameters

Query parameters are a way to pass additional data to the server via the URL.

query-params.js

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

app.get('/search', (req, res) => {
  res.send(`Search Query: ${req.query.q}`);
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

EXPLANATION:

const express = require('express'); First, we import the Express module.
const app = express(); We create an instance of an Express application.
app.get('/search', (req, res) => { ... }); We define a route that listens for GET requests at the /search endpoint.
const query = req.query.q; We access query parameters using req.query. In this example, req.query.q retrieves the value of the q parameter from the URL.
res.send(`Search Query: ${query}`); We send a response back to the client that includes the search query.
app.listen(3000, () => { ... }); Finally, we start the server and listen on port 3000.

Once the server is running, you can open your web browser and navigate to

http://localhost:3000/search?q=JavaScript

Expected Output

Search Query: JavaScript

Multiple Parameters

query-params.js

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

app.get('/search', (req, res) => {
  const query = req.query.q;
  const language = req.query.lang;
  res.send(`Search Query: ${query}, Language: ${language}`);
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

Once the server is running, you can open your web browser and navigate to

http://localhost:3000/search?q=JavaScript&lang=en

Search Query: JavaScript, Language: en

Middleware

Middleware functions are a core concept in Express.js, providing a powerful way to handle various aspects of the request-response cycle. Middleware functions have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.

What is Middleware?

Middleware functions are functions that can perform the following tasks:

Execute any code.
Make changes to the request and response objects.
End the request-response cycle.
Call the next middleware function in the stack.

If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

Types of Middleware

Application-Level MiddlewareBound to an instance of express.Router()
Router-Level Middleware The name of the function, used to call it.
Error-Handling MiddlewareHas four arguments (err, req, res, next)
Built-in MiddlewareProvided by Express.js, like express.static, express.json, and express.urlencoded
Third-Party MiddlewareCreamted by the community, like morgan, body-parser, cors, etc.

Using Middleware

using-middleware.js

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

// Custom middleware function
const logger = (req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
};

app.use(logger);

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

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

Built-in Middleware

Express comes with a few built-in middleware functions that you can use.

built-in-middleware.js

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

// Built-in middleware to parse JSON bodies
app.use(express.json());

// Built-in middleware to serve static files
app.use(express.static('public'));

app.post('/data', (req, res) => {
  res.send(`Received JSON data: ${JSON.stringify(req.body)}`);
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

Middleware functions provide a flexible and powerful way to handle the request-response cycle in Express applications. By using middleware, you can modularize your code and handle various tasks effectively.

Connecting to a Database

Using MongoDB with Mongoose

Mongoose is an Object Data Modeling (ODM) library for MongoDB and Node.js.

mongodb.js

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true });

const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', () => {
  console.log('Connected to the database');
});

// Defining a schema
const userSchema = new mongoose.Schema({
  name: String,
  age: Number
});

// Creating a model
const User = mongoose.model('User', userSchema);

// Creating a new user
const newUser = new User({ name: 'John Doe', age: 30 });
newUser.save((err) => {
  if (err) return console.error(err);
  console.log('User saved successfully');
});

Authentication and Authorization

Implementing JWT Authentication

JSON Web Tokens (JWT) are an open, industry-standard RFC 7519 method for representing claims securely between two parties.

authentication-authorization.js

const express = require('express');
const jwt = require('jsonwebtoken');
const app = express();
const port = 3000;

app.use(express.json());

const users = [{ id: 1, username: 'user', password: 'password' }];

app.post('/login', (req, res) => {
  const { username, password } = req.body;
  const user = users.find(u => u.username === username && u.password === password);
  if (user) {
    const token = jwt.sign({ userId: user.id }, 'your_jwt_secret');
    res.json({ token });
  } else {
    res.status(401).send('Invalid credentials');
  }
});

const authenticateJWT = (req, res, next) => {
  const token = req.header('Authorization').split(' ')[1];
  if (token) {
    jwt.verify(token, 'your_jwt_secret', (err, user) => {
      if (err) {
        return res.sendStatus(403);
      }
      req.user = user;
      next();
    });
  } else {
    res.sendStatus(401);
  }
};

app.get('/protected', authenticateJWT, (req, res) => {
  res.send('This is a protected route');
});

app.listen(port, () => {
  console.log(`Server running at http://localhost:${port}/`);
});

NOTES:

Express: A minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications.
Middleware:Functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.
Mongoose:An Object Data Modeling (ODM) library for MongoDB and Node.js.

FAQ

Q: What is Express?

A: Express is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications.

Q: How do I install Express?

A: You can install Express using NPM with the command npm install express.

Q: What is middleware in Express?

A: Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.

Q: How do I connect to MongoDB using Mongoose?

A: You can connect to MongoDB using Mongoose by installing it with npm install mongoose and using the mongoose.connect method with your MongoDB connection string.

Nodejs And BackendDatabase Integration

Testing And DebuggingUnit Testing