Express Error Handling Middleware

app.get('/files', (req, res, next) => {
  const fs = require('fs');
  
  fs.readFile('/does-not-exist.txt', (err, data) => {
    if (err) {
      // Passing an error to next() triggers the Error Handler
      return next(err); 
    }
    res.send(data);
  });
});

// Error-Handling Middleware MUST have exactly 4 arguments: (err, req, res, next)
// It must be defined AFTER all your other app.use() and routes!
app.use((err, req, res, next) => {
  console.error("Global Error Catcher:", err.stack);
  
  // Don't leak server details to the client in production
  const statusCode = err.status || 500;
  res.status(statusCode).json({
    error: "Something went wrong!",
    message: process.env.NODE_ENV === 'development' ? err.message : undefined
  });
});

// Express 4 doesn't catch errors thrown in 'async' functions automatically.
// You have to wrap them in try/catch and call next(err).
// A cleaner way is an async wrapper:

const asyncHandler = fn => (req, res, next) => {
  return Promise
    .resolve(fn(req, res, next))
    .catch(next); // Automatically passes caught errors to next()
};

// Now you don't need try/catch blocks!
app.get('/users', asyncHandler(async (req, res) => {
  const users = await db.getUsers(); // If this throws, it goes to the error handler
  res.json(users);
}));