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

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