What is Express.js and Why Use It?

The two programs below do the same job. Each one keeps a list of tasks in memory, returns the list for GET /tasks, and adds a task for POST /tasks. The first uses only the http module that ships with Node.js:

const http = require('node:http');

const tasks = [{ id: 1, title: 'Write the README', done: false }];

const server = http.createServer((req, res) => {
  const url = new URL(req.url, `http://${req.headers.host}`);

  if (req.method === 'GET' && url.pathname === '/tasks') {
    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify(tasks));
    return;
  }

  if (req.method === 'POST' && url.pathname === '/tasks') {
    let body = '';
    req.on('data', (chunk) => {
      body += chunk;
    });
    req.on('end', () => {
      let data;
      try {
        data = JSON.parse(body);
      } catch {
        res.writeHead(400, { 'Content-Type': 'text/plain' });
        res.end('Invalid JSON');
        return;
      }
      const task = { id: tasks.length + 1, title: data.title, done: false };
      tasks.push(task);
      res.writeHead(201, { 'Content-Type': 'application/json' });
      res.end(JSON.stringify(task));
    });
    return;
  }

  res.writeHead(404, { 'Content-Type': 'text/plain' });
  res.end('Not Found');
});

server.listen(3000);

The second uses Express:

const express = require('express');

const app = express();
app.use(express.json());

const tasks = [{ id: 1, title: 'Write the README', done: false }];

app.get('/tasks', (req, res) => {
  res.json(tasks);
});

app.post('/tasks', (req, res) => {
  const task = { id: tasks.length + 1, title: req.body.title, done: false };
  tasks.push(task);
  res.status(201).json(task);
});

app.listen(3000);

Both servers return the same tasks. A POST /tasks request with the body {"title":"Buy milk"} gets the status 201 Created and the body {"id":2,"title":"Buy milk","done":false} from either one. The first program needs 40 lines for that work, and the second needs 18.

What Express Did for You

The difference in length comes from four jobs that the first program does by hand and the second program leaves to Express.

Choosing the code that runs. The raw server compares req.method and the path in a chain of if statements. Express lets you register one function for each combination of method and path, such as app.get('/tasks', ...), and calls the matching function. A path with its method and its function is called a route, and the function is the route's handler.

Reading the request body. A request body arrives in pieces, so the raw server collects the pieces with data and end events and then parses them. In Express, the single line app.use(express.json()) does that work, and the handler finds the parsed object in req.body.

Writing the response. The raw server sets the status code and the Content-Type header itself, and converts the object to a string. In Express, res.json(task) converts the object, sets Content-Type: application/json; charset=utf-8, and sends it. The call res.status(201) sets the status code first.

Answering requests that match nothing. The raw server ends with its own 404 response. Express sends a 404 response automatically when no route matches, as this request to the Express version shows:

GET /nothing HTTP/1.1

HTTP/1.1 404 Not Found
Content-Type: text/html; charset=utf-8

<pre>Cannot GET /nothing</pre>

Express also rejects a body that is not valid JSON. When the body {"title": reaches express.json(), Express answers with 400 Bad Request before your handler runs. The response body is an HTML error page, and chapter 2 shows how to replace it with your own JSON error.

Express Runs on Top of the http Module

Express does not replace the Node.js http module. The object that express() returns is itself a function that takes a request and a response, (req, res), which is exactly the kind of function that http.createServer() accepts. When you call app.listen(3000), Express runs http.createServer(app) and starts that server on port 3000.

As a result, the req and res objects in an Express handler are the same Node.js request and response objects, with extra methods added. The method res.json() is an Express addition, while res.setHeader() and req.headers come from Node.js and still work.