Setup

Create & Start Server

const express = require("express");
const app = express();
app.listen(3000, () => console.log("Running on :3000"));

Built-in Middleware

app.use(express.json());          // parse JSON bodies
app.use(express.urlencoded({ extended: true })); // form data
app.use(express.static("public")); // serve static files

Routing

HTTP Methods

app.get("/users", (req, res) => res.json(users));
app.post("/users", (req, res) => res.status(201).json(req.body));
app.put("/users/:id", (req, res) => res.json(updated));
app.delete("/users/:id", (req, res) => res.sendStatus(204));

Route Parameters

app.get("/users/:id", (req, res) => {
  const { id } = req.params;
  res.json({ id });
});

Query Strings

// GET /search?q=express&page=2
app.get("/search", (req, res) => {
  const { q, page } = req.query;
  res.json({ q, page });
});

Middleware

Application-Level

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

Route-Level

const auth = (req, res, next) => {
  if (!req.headers.authorization) return res.sendStatus(401);
  next();
};
app.get("/secret", auth, (req, res) => res.json({ ok: true }));

Execution Order

app.use(fn)	Runs on every request (in order)
app.use(path, fn)	Runs only on matching path prefix
next()	Passes control to next middleware
next(err)	Skips to error handler

Request & Response

Request Object

req.params	Route parameters (`/users/:id`)
req.query	Query string (`?key=val`)
req.body	Parsed request body (needs parser)
req.headers	Request headers object
req.method	HTTP method (GET, POST, ...)
req.path	URL pathname
req.cookies	Cookies (needs cookie-parser)

Response Object

res.json(obj)	Send JSON response
res.send(body)	Send string/Buffer/object
res.status(code)	Set HTTP status (chainable)
res.redirect(url)	302 redirect (or pass status)
res.sendFile(path)	Send a file as response
res.sendStatus(code)	Send status with default text
res.set(header, val)	Set response header

Error Handling

Error Middleware

// Must have 4 parameters — Express recognizes it as error handler
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(err.status || 500).json({ error: err.message });
});

Define error handlers after all other app.use() and routes

Async Errors

// Wrap async route handlers to catch rejections
const wrap = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

app.get("/data", wrap(async (req, res) => {
  const data = await fetchData();
  res.json(data);
}));

Static Files

Serve Static Directory

app.use(express.static("public"));
// serves public/style.css at /style.css

// With virtual path prefix
app.use("/assets", express.static("public"));
// serves public/style.css at /assets/style.css

Options

dotfiles	`'ignore'` \| `'allow'` \| `'deny'`
maxAge	Cache-Control max-age in ms
index	Index file name (default: `index.html`)
fallthrough	Pass to next middleware on 404

Templates

View Engine Setup

app.set("view engine", "ejs");
app.set("views", "./views");

app.get("/", (req, res) => {
  res.render("index", { title: "Home", items: [1, 2, 3] });
});

Common Engines

ejs	Embedded JS templates (`<%= val %>`)
pug	Indentation-based (formerly Jade)
handlebars	Mustache-style (`{{val}}`)

Router

Modular Routes

// routes/users.js
const router = require("express").Router();
router.get("/", (req, res) => res.json(users));
router.get("/:id", (req, res) => res.json(user));
module.exports = router;

Mount Router

const usersRouter = require("./routes/users");
app.use("/api/users", usersRouter);
// GET /api/users -> router's "/"
// GET /api/users/5 -> router's "/:id"

Router Methods

router.get/post/put/delete	HTTP method handlers
router.use(fn)	Router-level middleware
router.param(name, fn)	Pre-process route param
router.route(path)	Chain methods on one path

Authentication Patterns

JWT Middleware

const jwt = require("jsonwebtoken");
const auth = (req, res, next) => {
  const token = req.headers.authorization?.split(" ")[1];
  if (!token) return res.sendStatus(401);
  req.user = jwt.verify(token, process.env.SECRET);
  next();
};

Protected Routes

app.get("/profile", auth, (req, res) => {
  res.json({ user: req.user });
});
app.use("/api/admin", auth, adminRouter);

Common Patterns

CORS

const cors = require("cors");
app.use(cors());                         // allow all origins
app.use(cors({ origin: "https://example.com" })); // restrict

Environment & Config

const port = process.env.PORT || 3000;
app.listen(port);

// Access env in routes
if (app.get("env") === "production") {
  app.use(helmet());
}

Useful npm Packages

cors	Cross-origin resource sharing
helmet	Security headers
morgan	HTTP request logger
cookie-parser	Parse Cookie header
dotenv	Load `.env` into `process.env`
multer	Multipart form data (file uploads)

EXPRESS.JS QUICK REFERENCE