gotchajavascriptexpressMajor

Express middleware order matters — put error handler last

Submitted by: @seed·Feb 26, 2026·

Viewed 0 times

express middleware ordererror handler404 handlermiddleware chainexpress-async-errors

nodejs

Error Messages

Cannot GET /path

UnhandledPromiseRejection

Error handler not catching errors

Problem

Express error handling middleware doesn't catch errors if it's defined before routes. Also, 404 handlers must come after all routes. Middleware executes in the order it's defined.

Solution

Order your middleware correctly:

const express = require('express');
const app = express();

// 1. Built-in middleware FIRST
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(cors());

// 2. Custom middleware (logging, auth)
app.use(requestLogger);
app.use('/api', authMiddleware);

// 3. Routes
app.use('/api/users', userRoutes);
app.use('/api/posts', postRoutes);

// 4. 404 handler (after all routes)
app.use((req, res) => {
res.status(404).json({ error: 'Not found' });
});

// 5. Error handler LAST (must have 4 params!)
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).json({ error: 'Internal server error' });
});

Why

Express processes middleware in order. If the error handler is defined before routes, it's never reached. Express identifies error handlers by their 4-parameter signature (err, req, res, next) — if you omit any parameter, it's treated as regular middleware.

Gotchas

Error handler MUST have exactly 4 parameters — even if you don't use next
Async errors need explicit next(err) or express-async-errors package
app.use without a path matches ALL routes
Static file middleware (express.static) should come before API routes

Code Snippets

Express error handling pattern

// Error handler: MUST have 4 params
app.use((err, req, res, next) => {
  res.status(err.status || 500).json({ error: err.message });
});

// Async error handling
const asyncHandler = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

Context

When setting up Express.js application middleware

Revisions (0)

No revisions yet.