Express Intro & Routing

Express is the most popular web framework for Node.js — a thin layer over the http module that gives you a clean API for defining routes, handling requests, and sending responses with far less boilerplate.

Learn Express Intro & Routing in our free Node.js course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick…

Part of the free Node.js course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll have installed Express, written a server that responds to a request, defined routes for different HTTP methods and URLs, read route parameters and query strings, and sent back both plain text and JSON with the right status codes.

What You'll Learn in This Lesson

1️⃣ Installing Express & Your First Server

Express isn't built into Node — it lives on npm, the package registry. From inside your project folder you install it once, and it lands in node_modules so your code can require it.

Now the classic "hello server". Read every line below — it builds an app, defines one route, and starts listening. Save it as server.js and run node server.js .

2️⃣ Routing: Methods, Params & Queries

A route is the pairing of an HTTP method and a URL path with the function that handles it. Express gives you one method per verb — app.get , app.post , app.put , app.delete — and two ways to read variable data from the URL:

• • Route parameters — a segment starting with : like /users/:id is captured into req.params.id .
• • Query strings — the part after ? like /search?q=cats shows up parsed in req.query.q .

Your turn. The route below is almost done — fill in the two blanks marked ___ using the 👉 hints so a request to /ping returns JSON.

3️⃣ Sending Responses

The response object res is how you reply. The three you'll reach for constantly are res.send (text or HTML), res.json (data as JSON), and res.status (the HTTP status code, chained before a body). Each route may send exactly one response.

Mini-Challenge: A Tiny Greeting API

No blanks this time — just a brief and an outline. Write the two routes yourself, run the server, and check your output against the example in the comments. This is exactly the kind of small API that builds real fluency.

Common Errors (and the fix)

• "Cannot find module 'express'" — You forgot to install it, or you're in the wrong folder. Run npm install express inside your project (the one with package.json ) before running the file.
• Program exits immediately / nothing happens — You forgot app.listen(3000) . Without it the server is never started, so it has nothing to do and Node exits.
• "Cannot set headers after they are sent to the client" — You sent two responses in one handler (for example res.send(...) then res.json(...) ). Send exactly once, and return after sending so the rest of the handler doesn't run.
• req.params is undefined for your value — The key must match the route. /users/:id gives you req.params.id , not req.params.userId . The param name and the property name are the same word.
• A specific route never runs — Route order matters. A broad route like /users/:id declared first will shadow /users/new . Put literal, specific routes before parameterised ones.

Pro Tips

• 💡 Use res.json() for APIs: it sets the application/json header explicitly, making your intent clear to both clients and reviewers.
• 💡 Always return after sending: writing return res.status(404).json(...) prevents the dreaded "headers already sent" crash.
• 💡 Try a REST client like the browser, curl , or an extension to test POST/PUT/DELETE, which a browser address bar can't send on its own.

📋 Quick Reference — Express Basics

Concept

Express Syntax

Create the app

const app = express()

Define a GET route

app.get(path, fn)

Read a route param

req.params.id

Read a query value

req.query.q

Send text

res.send(x)

Send JSON

res.json(obj)

Set the status code

res.status(404)

Start the server

app.listen(3000)

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ Express is a thin layer over http installed with npm install express
• ✅ const app = express() creates your server; app.listen(3000) starts it
• ✅ app.get/post/put/delete define routes per HTTP method
• ✅ req.params reads route params; req.query reads query strings
• ✅ res.send , res.json , and res.status shape the response — send exactly one
• ✅ Route order matters — put specific routes before parameterised ones
• ✅ Next lesson: Middleware

Practice quiz

Continue this course

• Previous: Error Handling
• Next: Middleware