```typescript
/** The factorial of n. */
export function factorial(n: number): number {
  if (n < 0) {
    throw new Error('n must be non-negative');
  } else if (!Number.isInteger(n)) {
    throw new Error('n must be an integer');
  }

  return n === 0 || n === 1 ? 1 : n * factorial(n - 1);
}
```