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

  let sum = 0;
  for (const digit of code) {
    sum += parseInt(digit, 10);
  }

  return sum % 10 === 0 ? 0 : 10 - sum % 10;
}
```