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