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