Express basics

What you'll learn

How to install Express, define routes for different URLs, read route parameters and query strings, and send text or JSON back.

Why it matters

The built-in http module makes you check URLs and methods by hand. Express is the most popular Node web framework: it turns that into short, readable route definitions, and is the base that many other tools build on.

Explanation

Install it in your project:

An Express app is made of routes. Each route says: for this method and this path, run this function.

• app.get, app.post, app.put, app.delete match HTTP methods.
• req.params holds route parameters from the path: /users/:id gives req.params.id.
• req.query holds the query string: /search?q=node gives req.query.q.
• res.send("text") sends text or HTML, res.json(object) sends JSON, and res.status(404) sets the status code.
• app.listen(port, callback) starts the server.

Example

Save as app.mjs (this is a real server that keeps running until you press Ctrl+C):

Step-by-step walkthrough

1. express() creates the app.
2. The / route answers with plain text.
3. /hello/:name captures whatever is in the :name position. Visiting /hello/Asha sends Hello, Asha!.
4. /search reads ?q=... from the URL and replies with JSON. Visiting /search?q=node gives {"query":"node"}, and /search alone gives {"query":null}.
5. listen starts the server and prints a message.

Try it yourself

The script below starts the server, calls its own routes with fetch, prints the replies and shuts down, so you can see everything in one run. Save it as try.mjs, run npm install express, then node try.mjs.

try.mjsNode.js

Editable codeimport express from "express"; const app = express(); app.get("/", (req, res) => res.send("Hello from Express")); app.get("/hello/:name", (req, res) => res.send(`Hello, ${req.params.name}!`)); app.get("/search", (req, res) => res.json({ query: req.query.q ?? null })); const server = app.listen(3000, async () => { console.log("Listening on http://localhost:3000"); const base = "http://localhost:3000"; console.log(await (await fetch(base + "/")).text()); console.log(await (await fetch(base + "/hello/Asha")).text()); console.log(await (await fetch(base + "/search?q=node")).text()); console.log(await (await fetch(base + "/search")).text()); server.close(); });

Show output ResetCopyCtrl+Enter to run

Common mistakes

• Forgetting npm install express, which gives "Cannot find package".
• Never sending a response. Every route must call res.send, res.json or res.end.
• Putting a general route before a specific one. Express uses the first match.
• Port already in use. Stop the old server or choose another port.

Quick quiz

Summary

• Express maps methods and paths to functions.
• req.params for path values, req.query for query strings.
• res.send and res.json reply; app.listen starts the server.

Next: building a REST API.

npm init -y
npm install express

app.get("/path", (req, res) => { ... });

import express from "express";

const app = express();

app.get("/", (req, res) => {
  res.send("Hello from Express");
});

app.get("/hello/:name", (req, res) => {
  res.send(`Hello, ${req.params.name}!`);
});

app.get("/search", (req, res) => {
  res.json({ query: req.query.q ?? null });
});

app.listen(3000, () => {
  console.log("Listening on http://localhost:3000");
});