```
/** The factorial of n. */
export function factorial(n: number): number {
  // Check if input is non-integer or negative
  if (!Number.isInteger(n) || n < 0) {
    throw new Error("Factorial is not defined for negative numbers or non-integers.");
  }
  // Base case when n is 0
  return n === 0 ? 1 : n * factorial(n - 1);
}
```