Error-Handling Middleware

The 4-argument signature

A middleware with exactly (err, req, res, next) is treated as an error handler. Express skips straight to it whenever next(err) is called anywhere.

A centralized handler

Register it last, after all routes — one place to log errors and shape the JSON response.

app.get('/risky', (req, res, next) => {
  try {
    throw new Error('Something broke');
  } catch (err) {
    next(err);   // hand off to the error handler
  }
});

// must be registered after all routes
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: err.message || 'Internal Server Error' });
});

Quick check: How many arguments must an error-handling middleware function accept?

• 2
• 3
• 4