Handling Requests and Responses

1. Introduction

The goal of this tutorial is to teach you how to handle HTTP requests and responses using Express, a flexible Node.js web application framework. By the end of this tutorial, you will learn how to extract information from requests, send different types of responses, and understand the basics of how data is transferred between client and server.

Prerequisites

For this tutorial, you should have a basic understanding of:
- JavaScript, as Express is a Node.js framework
- Node.js and npm (Node Package Manager)
- HTTP (Hypertext Transfer Protocol)

2. Step-by-Step Guide

Every interaction between a client (usually a web browser) and a server involves sending a request and receiving a response. In Express, these are represented by the Request (req) and Response (res) objects.

HTTP Requests

A HTTP request contains all the information a server needs to understand and respond to it. This includes the request method (GET, POST, etc.), the URL, headers, and any data sent by the client.

HTTP Responses

A HTTP response is what the server sends back to the client. It includes a status code, headers, and any data the server wants to send back.

In Express, you handle requests with middleware functions, which have access to the req and res objects and a next function. You define these functions and then tell Express to use them.

3. Code Examples

Here's a basic example of an Express server that responds to GET requests at the root URL (/).

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

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

app.listen(3000, () => {
  console.log('Server listening on port 3000');
});

• app.get tells Express to use the provided function for all GET requests to the / URL.
• (req, res) => {} is the function that handles these requests.
• res.send sends a response back to the client.

When you run this server and visit http://localhost:3000 in your browser, you should see "Hello, world!".

4. Summary

In this tutorial, you've learned how to handle HTTP requests and responses in Express, how to extract information from requests, and how to send responses. You've also seen how to define middleware functions and use them in Express.

Next Steps

To learn more about Express and its features, check out the official Express.js documentation.

5. Practice Exercises

1. Create an Express server that responds to POST requests at the /data URL, and responds with the same data it receives.
2. Modify the server from exercise 1 to respond with a status code of 418 ("I'm a teapot") instead of the default 200.

Solutions

1. Here's how you could create the server for exercise 1:

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

app.use(express.json()); // for parsing application/json

app.post('/data', (req, res) => {
  res.json(req.body);
});

app.listen(3000, () => {
  console.log('Server listening on port 3000');
});

1. And here's how you could modify it for exercise 2:

app.post('/data', (req, res) => {
  res.status(418).json(req.body);
});

res.status(418) sets the status code of the response to 418. You can use any valid HTTP status code here.