/**
 * Computes the factorial of a non‑negative integer.
 *
 * @param {number} n - The number to compute the factorial of.
 * @returns {number|undefined} The factorial of n for valid inputs, otherwise undefined.
 */
export function factorial(n) {
  if (typeof n !== "number" || !Number.isFinite(n)) return undefined;
  if (!Number.isInteger(n)) return undefined;
  if (n < 0) return undefined;

  let result = 1;
  for (let i = 2; i <= n; i++) {
    result *= i;
  }
  return result;
}