Errors in Node and Express

Error bugs are sneaky because the happy path works. The tests that fail are the ones that send bad input and expect a specific status and message.

Throw real errors

throw "not found";                 // ❌ a string: no stack, no .message, instanceof fails
throw new Error("not found");      // ✅

A small error hierarchy

class HttpError extends Error {
  constructor(status, message) {
    super(message);
    this.status = status;
    this.name = this.constructor.name;
  }
}
class NotFound extends HttpError { constructor(what) { super(404, `${what} not found`); } }
class BadRequest extends HttpError { constructor(msg) { super(400, msg); } }

One error handler then maps every error to a response:

app.use((err, req, res, next) => {               // four args, registered LAST
  const status = err.status ?? 500;
  res.status(status).json({ error: status === 500 ? "Internal error" : err.message });
});

Async handlers in Express 4

Express 4 only catches errors thrown synchronously. A rejected promise in an async handler never reaches the error middleware: the request hangs or the process logs an unhandled rejection.

// ❌ rejection escapes Express 4
app.get("/items/:id", async (req, res) => {
  const item = await Item.findById(req.params.id);
  res.json(item);
});

// ✅ forward it
const wrap = (fn) => (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next);
app.get("/items/:id", wrap(async (req, res) => { … }));

Express 5 forwards rejected promises for you, but labs and many real codebases still run Express 4.

Other classics

• Respond once. if (!user) res.status(404).json(…) without return keeps going and sends a second response: "Cannot set headers after they are sent".
• Don't swallow. catch (e) {} turns a 500 into a confusing 200 with an empty body.
• Don't leak internals. Return a clean message for 500s; log the stack server-side.
• Wrong class of status. Validation errors are 400, missing records 404, duplicates 409. A 500 in a test usually means an error wasn't mapped.
• Unhandled rejections crash Node 15+ by default. That's a feature: it surfaces the bug.