10. Error handling: try/catch, custom Error subclasses

class ValidationError extends Error {
  constructor(message, field) {
    super(message);
    this.name = "ValidationError";
    this.field = field;
  }
}

function validateAge(age) {
  if (age < 0) throw new ValidationError("Age cannot be negative", "age");
  return age;
}

try {
  validateAge(-5);
} catch (err) {
  if (err instanceof ValidationError) {
    console.error(`Invalid ${err.field}: ${err.message}`);
  } else {
    throw err;   // re-throw anything you didn't expect — never swallow silently
  }
} finally {
  console.log("validation attempted");   // always runs, even after a return/throw
}

Never write an empty catch {} — it hides real bugs. If you genuinely don't care about an error, catch it and log it, so a silent failure is at least discoverable in production logs.