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