```typescript
/**
 * The factorial of n.
 */
export function factorial(n: number): number {
  if (typeof n !== 'number' || n < 0) {
    throw new Error('n must be a non-negative integer');
  }
  if (!Number.isInteger(n)) {
    @ts-ignore
    throw new Error(`n must be an integer, not ${ typeof n}`);
  }

  if (n === 0) {
    return 1;
  }
  return Number(n) * factorial(Number(n) - 1);
}
```