How do you create custom Error types in JavaScript?

class ValidationError extends Error {
  constructor(message, field) {
    super(message);                    // sets .message and .stack
    this.name = 'ValidationError';     // override — default is 'Error'
    this.field = field;                // custom property
  }
}

class NetworkError extends Error {
  constructor(message, statusCode) {
    super(message);
    this.name = 'NetworkError';
    this.statusCode = statusCode;
  }
}

// Usage — instanceof lets you catch specific types
function validate(user) {
  if (!user.name)  throw new ValidationError('Name required',  'name');
  if (!user.email) throw new ValidationError('Email required', 'email');
}

try {
  validate({ name: '' });
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(`Field "${err.field}": ${err.message}`);
  } else if (err instanceof NetworkError) {
    console.log(`HTTP ${err.statusCode}: ${err.message}`);
  } else {
    throw err; // re-throw unknown errors — don't swallow them
  }
}