```ts
/** The factorial of n. */
export function factorial(n: number): number {
  if (typeof n !== 'number' || n < 0) {
    throw new Error('Factorial is not defined for negative numbers');
  } else if (!Number.isInteger(n)) {
    throw new Error('Factorial is only defined for integers');
  }
  return n === 0 ? 1 : n * factorial(n - 1);
}
```