Express Basics

Node.js · Reference cheat sheet

📋 Overview

Express is a minimal Node HTTP framework: an app, middleware stack, and route handlers. Widely used and flexible; you wire most pieces yourself.

API	Role
`express()`	Create app
`app.get/post/...`	Register route + method
`app.use`	Mount middleware or sub-app
`req` / `res`	Incoming request / outgoing response
`next`	Pass control to next middleware

Minimal app:

import express from "express";

const app = express();
app.get("/", (req, res) => res.json({ ok: true }));
app.listen(9000);

JSON body parsing:

app.use(express.json());

app.post("/users", (req, res) => {
  res.status(201).json(req.body);
});

404 fallback:

app.use((req, res) => {
  res.status(404).json({ error: "Not found" });
});

Forgetting express.json() leaves req.body undefined for JSON POSTs.
Error-handling middleware must have 4 args (err, req, res, next) or Express skips it.
Listening on a port already in use crashes at startup — check availability (9000+).