express-error-handler

name	express-error-handler
description	Diagnose and fix Express.js error handling issues across all apps (FlowInCash, Crispi, MicroApps)
version	1.0.0
tags	["express","error-handling","node","typescript"]

Express Error Handler

Use this skill when an Express app returns 500 errors, crashes on unhandled rejections, or logs error stack traces.

Applies To

• FlowInCash (/media/bob/I/AI_Projects/FlowInCash)
• Crispi-app (/media/bob/I/AI_Projects/Crispi-app)
• FliC-MicroApps (/media/bob/I/AI_Projects/FliC-MicroApps)
• Crispi-MicroApps (/media/bob/I/AI_Projects/Crispi-MicroApps)

Diagnosis Steps

1. Read the error log — extract the stack trace and HTTP status code
2. Classify the error:
   • TypeError / ReferenceError → code bug, likely missing null check
   • ECONNREFUSED / ETIMEDOUT → downstream service (DB, Plaid, AWS) is down
   • ValidationError / ZodError → bad input from client
   • ENOMEM / EMFILE → resource exhaustion
3. Locate the handler — search src/routes/ for the failing endpoint
4. Check the middleware chain — errors may be swallowed before reaching the error handler

Fix Patterns

Missing async error propagation

// BAD — unhandled promise rejection
router.get('/accounts', async (req, res) => {
  const data = await fetchAccounts(); // throws, not caught
  res.json(data);
});

// GOOD — wrap or use express-async-errors
router.get('/accounts', async (req, res, next) => {
  try {
    const data = await fetchAccounts();
    res.json(data);
  } catch (err) {
    next(err);
  }
});

Error handler not last in middleware chain

app.use('/api', routes);
app.use(errorHandler); // Must be LAST

Leaking internal errors to clients

// BAD
res.status(500).json({ error: err.message, stack: err.stack });

// GOOD
res.status(500).json({ error: 'Internal server error', requestId: req.id });
logger.error({ err, requestId: req.id, path: req.path });

Validation

1. Run npm test in the affected project
2. Trigger the failing request to confirm the fix
3. Check that error logs show structured output, no raw stack traces to client