How do you handle errors in a Node.js backend?

Errors fall into two groups. Operational errors, such as a validation failure or a timeout, are expected and should produce a controlled response. Programmer errors, such as reading a property of undefined, indicate a bug and should not be silently swallowed. The handling strategy differs.

app.get("/users/:id", async (req, res, next) => {
  try {
    const user = await users.findById(req.params.id);
    if (!user) {
      return res.status(404).json({ error: "not_found" });
    }
    res.json(user);
  } catch (error) {
    next(error);
  }
});

app.use((error, req, res, next) => {
  req.log.error({ error }, "request failed");
  res.status(error.status || 500).json({ error: "internal_error" });
});

Rules that prevent incidents

• Never send stack traces or internal messages to clients
• Attach context when rethrowing, including request or job identifiers
• Use error codes or typed errors instead of string matching
• Give every network call a timeout and treat cancellation as a normal outcome
• Do not retry non-idempotent operations without an idempotency key

At the process level, listen for unhandledRejection and uncaughtException, log the failure, stop accepting new work and exit. Continuing after an uncaught exception leaves the process in an unknown state, and a supervisor should restart it. Express 5 forwards rejected async handlers automatically, but explicit try and catch still makes intent clear.