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