```ts


/** The factorial of n. */
export function factorial(n: number): number {
  // Early return for non-integer or negative inputs
  if (!Number.isInteger(n) || n < 0) {
    throw new Error("Input must be a non-negative integer.");
  }
  // Base case for factorial(0)
  return (n === 0 && n) ? 1 : n * factorial(n - 1);
}
```