```ts
function checkDigit(code: string): number {
  if (!code || !/^\d+$/.test(code)) {
    throw new Error('Invalid code');
  }

  const sum = Array.from(code).reduce((acc, digit) => acc + parseInt(digit), 0);
  return (sum % 10 === 0 ? 0 : 10 - sum % 10);
}
```