05 - Introduction to Express.js

• nodejs
• by Sn0wAlice

Introduction

Express.js is a lightweight and flexible Node.js framework that simplifies building web applications and APIs. It provides powerful tools for handling routing, middleware, and HTTP requests efficiently.

---

1. Installing Express.js

Before using Express, install it using npm:

npm install express

To create a basic Express application, require the module:

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

---

2. Creating a Basic Express Server

Express makes it easy to create an HTTP server.

Example:

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

app.get("/", (req, res) => {
    res.send("Hello from Express!");
});

app.listen(3000, () => console.log("Express server running on port 3000"));

Explanation:

• .get("/") handles GET requests to the root URL.
• res.send() sends a response.
• The server listens on port 3000.

---

3. Handling Different HTTP Methods

Express allows handling different HTTP methods such as GET, POST, PUT, and DELETE.

Example:

app.get("/users", (req, res) => {
    res.json({ message: "GET request received" });
});

app.post("/users", (req, res) => {
    res.json({ message: "POST request received" });
});

app.put("/users/:id", (req, res) => {
    res.json({ message: `User ${req.params.id} updated` });
});

app.delete("/users/:id", (req, res) => {
    res.json({ message: `User ${req.params.id} deleted` });
});

---

4. Middleware in Express

Middleware functions are executed during request processing. They can be used for logging, authentication, and modifying request objects.

Using Middleware

app.use((req, res, next) => {
    console.log(`${req.method} request received at ${req.url}`);
    next();
});

Built-in Middleware

app.use(express.json()); // Parses JSON requests
app.use(express.urlencoded({ extended: true })); // Parses URL-encoded data

---

5. Handling Static Files

Express can serve static files like images, stylesheets, and JavaScript files.

app.use(express.static("public"));

Place files in a public directory, and they can be accessed directly in the browser.

---

6. Working with URL Parameters and Query Strings

Express makes handling URL parameters and query strings simple.

Example:

app.get("/user/:id", (req, res) => {
    res.send(`User ID: ${req.params.id}`);
});

Handling Query Strings

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

Access this via http://localhost:3000/search?term=Node.js.

---

7. Error Handling in Express

Express provides error-handling middleware to catch errors and respond properly.

Example:

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

---

8. Setting Up a REST API with Express

Example:

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

const users = [
    { id: 1, name: "Alice" },
    { id: 2, name: "Bob" }
];

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

app.post("/users", (req, res) => {
    const newUser = { id: users.length + 1, name: req.body.name };
    users.push(newUser);
    res.status(201).json(newUser);
});

app.listen(3000, () => console.log("API server running on port 3000"));

---

🏆 Exercises

• 1. Create a basic Express server that responds with "Hello, Express!".
• 2. Modify the server to handle different HTTP methods for a /users route.
• 3. Implement a middleware function that logs request details.
• 4. Create a route that serves static files from a directory.

5. Build a simple REST API with GET, POST, and DELETE routes.

---

Conclusion

Express.js is a powerful framework that simplifies building web applications and APIs. This chapter covered how to create a server, handle routes, use middleware, and set up a REST API.

---