```ts
/** The factorial of n. */
export function factorial(n: number): number {
  if (typeof n !== 'number' || n % 1 !== 0 || n < 0) {
    throw new Error('Factorial is not defined for non-integer or negative numbers.');
  }
  if (n === 0) {
    return 1;
  }
  return n * factorial(n - 1);
}
```