function isValidHexColor(hex) {
  // Matches 3-digit or 6-digit hex color codes, with or without a leading #
  const hexRegex = /^#?([0-9A-Fa-f]{3}){1,2}$/;
  return hexRegex.test(hex);
}

// Example usage:
console.log(isValidHexColor("#FFF")); // true
console.log(isValidHexColor("FFF"));  // true
console.log(isValidHexColor("#FF00FF")); // true
console.log(isValidHexColor("FF00FF")); // true
console.log(isValidHexColor("#12345")); // false (5 digits)
console.log(isValidHexColor("invalid")); // false