Express.js Cheat Sheet

Covers building routes, applying middleware, handling errors in async handlers, and key Express request/response methods.

2 PagesIntermediateFeb 28, 2026

Basic App & Routing

Setting up routes and parsing JSON bodies.

javascript

const express = require('express');const app = express();app.use(express.json()); // parse JSON request bodiesapp.get('/users/:id', (req, res) => {  res.json({ id: req.params.id });});app.post('/users', (req, res) => {  const { name } = req.body;  res.status(201).json({ name });});app.listen(3000, () => console.log('Server running on port 3000'));

Middleware

Custom, router-level, and third-party middleware.

javascript

// Custom middlewarefunction logger(req, res, next) {  console.log(`${req.method} ${req.url}`);  next(); // pass control to the next handler}app.use(logger);// Router-level middlewareconst router = express.Router();router.use((req, res, next) => {  if (!req.headers.authorization) return res.sendStatus(401);  next();});router.get('/profile', (req, res) => res.json({ ok: true }));app.use('/api', router);// Third-party middlewareconst cors = require('cors');app.use(cors());

Error Handling & Async Routes

Forwarding errors to Express's error middleware.

javascript

// Async route (Express 4 does not catch async errors automatically)app.get('/data', async (req, res, next) => {  try {    const data = await fetchData();    res.json(data);  } catch (err) {    next(err); // forward to error-handling middleware  }});// Error-handling middleware must declare all 4 argumentsapp.use((err, req, res, next) => {  console.error(err.stack);  res.status(500).json({ error: 'Internal Server Error' });});

Key Methods & Concepts

Frequently used request/response APIs.

app.use(path, fn)- mounts middleware, optionally scoped to a path prefix
req.params / req.query / req.body- route params, query string, and parsed request body
res.status().json()- chainable response builder for setting status and sending JSON
express.static('public')- serves static files from a directory
next(err)- skips remaining route middleware and jumps to error-handling middleware
app.route(path)- chains multiple HTTP method handlers for a single path

Pro Tip

In Express 4.x, unhandled promise rejections inside async route handlers are not caught automatically: always wrap async handlers in try/catch and call next(err), or use a helper like express-async-handler (Express 5 fixes this natively).

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