HTTP Module

TL;DR

The built-in http module allows Node.js to transfer data over the Hyper Text Transfer Protocol. It provides the foundation for creating web servers and making outbound requests without needing external libraries like Express or Axios.

Mental Model

How It Works

The http module is built on top of Node’s net (TCP) module.

• http.createServer() creates a server instance.
• For every incoming connection, the server emits a 'request' event, passing an IncomingMessage (Request) and a ServerResponse (Response).
• Both the Request and Response objects are Streams. You read data from the request in chunks, and write data to the response in chunks.

Example

const http = require('http');

const server = http.createServer((req, res) => {
    // Basic routing
    if (req.url === '/' && req.method === 'GET') {
        res.writeHead(200, { 'Content-Type': 'application/json' });
        res.end(JSON.stringify({ message: 'Hello World!' }));
    } else {
        res.writeHead(404, { 'Content-Type': 'text/plain' });
        res.end('Not Found');
    }
});

server.listen(3000, () => console.log('Server running on port 3000'));

Common Interview Questions

If we have Express, why do we need to know the http module?

Express is a framework built on top of the http module. Under the hood, Express’s app is just a giant callback function passed to http.createServer(app). Understanding http helps you understand how Express handles streams, headers, and chunked encoding.

How do you read the body of a POST request using only the http module?

Because the request is a Readable stream, you must listen to the 'data' and 'end' events to collect the chunks into an array, concatenate them, and parse the resulting string.