```
/**
 * The factorial of n. Throws an Error if n is negative or non-integer.
 */
export function factorial(n: number): number {
  if (!Number.isInteger(n) || n < 0) {
    throw new Error("n must be a positive integer");
  }
  if (n === 0) {
    return 1;
  }
  return n * factorial(n - 1);
}
```