Error Handling | Engineering Interview Kit

TL;DR

Proper error handling in Node.js prevents the server from crashing unexpectedly and ensures clients receive meaningful HTTP status codes. Errors are typically divided into Operational Errors (expected, like “File not found”) and Programmer Errors (unexpected bugs, like reading property of undefined).

Mental Model

How It Works

Synchronous Code: Handled using standard try...catch blocks.
Asynchronous (Callbacks): Handled using the Error-First callback pattern (if (err) return cb(err)).
Asynchronous (Promises): Handled using .catch() or try...catch combined with async/await.

You should define a custom AppError class to distinguish your expected operational errors from random Node.js/V8 crashes.

Example

// Custom Error Class
class AppError extends Error {
  constructor(message, statusCode) {
    super(message);
    this.statusCode = statusCode;
    this.isOperational = true; // Marks this as an expected error
    Error.captureStackTrace(this, this.constructor);
  }
}

// In an Express Route
app.get('/user/:id', async (req, res, next) => {
  try {
    const user = await db.getUser(req.params.id);
    if (!user) throw new AppError('User not found', 404);
    res.json(user);
  } catch (err) {
    next(err); // Pass to global error handler
  }
});

Common Interview Questions

Should you ever swallow errors?

No. An empty catch(e) {} block (swallowing) makes debugging impossible. At the very least, log the error.

How do you handle errors centrally in Express?

You create a middleware function with exactly four arguments (err, req, res, next). Express recognizes this specific signature as the global error handler. All errors passed to next(err) will end up here, allowing you to format a consistent JSON response.