Creating Your First Express Server

Why it exists

Node.js provides low-level HTTP modules to build web servers, but they are verbose and require manually parsing URLs, headers, and request bodies. Express was created to provide a minimal, unopinionated layer on top of Node's native HTTP server to simplify routing, middleware, and response handling.

The mental model

Think of express() as a factory that creates a request-handling machine. You give this machine instructions (routes like app.get()) for what to do when a request arrives. Finally, you turn the machine on and connect it to a network port using app.listen(), so it can start receiving those requests.

How it works

The process involves three main steps. First, you import the Express library using const express = require('express'). Second, you create an application instance by calling the top-level express() function: const app = express(). This app object has methods for routing HTTP requests. Third, you define at least one route, like app.get('/', (req, res) => { ... }), which tells the app how to respond to a GET request at the root URL. The req and res objects are the same ones provided by Node.js, just enhanced by Express. Finally, you bind the application to a port and start the server with app.listen(port, callback), which makes it run continuously, waiting for connections.

When to use it

This is the fundamental starting point for any project using the Express framework. You will use this pattern to create REST APIs, traditional web applications with server-side rendering, or simple microservices that need to communicate over HTTP. It is the "File > New Project" for backend JavaScript development.

When not to use it

You might not use this basic setup directly if you're using a higher-level framework built on top of Express, like NestJS, which handles this boilerplate for you. For a full application, you would also typically use the Express generator to create a more structured project with separate files for routes and views, rather than putting everything in one file.

One canonical example

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

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

app.listen(port, () => {
console.log(Example app listening on port ${port});
});

This code creates a server that listens on port 3000. When you visit http://localhost:3000/ in a browser, it responds with "Hello World!". For any other path, it returns a 404 Not Found.