Creating a Server and Handling Requests

With Express installed, let’s build a server that actually handles requests — understanding ports, HTTP fundamentals, and how to send proper responses.

How HTTP works

When a client (browser, React app, curl) makes a request:

1. It opens a TCP connection to your server’s IP address and port
2. Sends an HTTP request: method + path + headers + optional body
3. Your server handles the request and sends an HTTP response: status code + headers + body
4. The connection closes (or is reused for keep-alive)

Your Express server listens on a port for incoming connections.

Ports

A port is a number that identifies a specific service on a machine. Common conventions:

• 80 — HTTP (production web servers, no port shown in URLs)
• 443 — HTTPS
• 3000, 3001, 8080 — common development ports

In development, you access your API at http://localhost:3000. In production on Railway, the PORT is assigned dynamically via process.env.PORT.

const PORT = parseInt(process.env.PORT || '3000', 10)

app.listen(PORT, () => {
  console.log(`Server listening on port ${PORT}`)
})

Sending JSON responses

res.json() serializes an object to JSON and sets Content-Type: application/json:

app.get('/posts', (req, res) => {
  res.json([
    { id: 1, title: 'My first post' },
    { id: 2, title: 'Another post' },
  ])
})

HTTP status codes

Status codes tell the client what happened:

Range	Meaning	Examples
2xx	Success	200 OK, 201 Created, 204 No Content
4xx	Client error	400 Bad Request, 401 Unauthorized, 404 Not Found
5xx	Server error	500 Internal Server Error

Set the status with res.status():

app.post('/posts', (req, res) => {
  // ... create post
  res.status(201).json({ id: newPost.id, title: newPost.title })
})

app.get('/posts/:id', (req, res) => {
  const post = findPost(req.params.id)
  if (!post) {
    return res.status(404).json({ error: 'Post not found' })
  }
  res.json(post)
})

Response methods

Method	Use for
res.json(object)	JSON API responses
res.send(text)	Plain text or HTML
res.status(code).json(object)	Response with explicit status code
res.sendStatus(code)	Status code with default message body
res.redirect(url)	Redirect to another URL

Reading the request body

To read JSON sent in a POST/PUT request body, add express.json() middleware:

app.use(express.json())

app.post('/posts', (req, res) => {
  const { title, body } = req.body
  console.log(title, body)
  res.status(201).json({ message: 'Created' })
})

Without express.json(), req.body is undefined.

Testing with curl

Test your API from the terminal without a browser:

# GET request
curl http://localhost:3000/posts

# POST request with JSON body
curl -X POST http://localhost:3000/posts \
  -H "Content-Type: application/json" \
  -d '{"title": "Hello", "body": "My first post"}'

Or use a GUI tool like Thunder Client (VS Code extension) or Postman.

Exercise

1. Add app.use(express.json()) to your server.
2. Create a POST /posts route that reads title and body from req.body and returns them with a 201 status.
3. Test with curl or Thunder Client — send {"title": "Hello", "body": "World"}.
4. What happens if you send a request without Content-Type: application/json? What is req.body?

Recap

• app.listen(PORT, callback) starts the HTTP server.
• Use process.env.PORT for the port — hosting platforms assign it dynamically.
• res.json(object) sends a JSON response with the correct Content-Type.
• res.status(code).json(object) sends a response with an explicit status code.
• app.use(express.json()) parses JSON request bodies into req.body.