```typescript
/** The factorial of n. */
export function factorial(n: number): number {
  if (typeof n !== 'number') {
    throw new Error('n must be a number');
  }
  if (n < 0) {
    throw new Error('n must be non-negative');
  }
  return n === 0 || n === 1 ? 1 : n * factorial(n - 1);
}
```