Luhn Checksum Validation for Card Numbers

export function isValidCardNumber(input) {
  const digits = normalize(input);
  if (digits.length < 12 || digits.length > 19) return false;
  return luhnSum(digits) % 10 === 0;
}

export function cardCheckDigit(partial) {
  const digits = normalize(partial);
  if (digits.length < 11 || digits.length > 18) {
    throw new RangeError("partial number must contain 11 to 18 digits");
  }
  const remainder = luhnSum(`${digits}0`) % 10;
  return String((10 - remainder) % 10);
}

function normalize(input) {
  if (typeof input !== "string" && typeof input !== "number") {
    throw new TypeError("card number must be a string or number");
  }
  const value = String(input).replace(/[\s-]/g, "");
  if (!/^\d*$/.test(value)) {
    throw new Error("card number may only contain digits, spaces and hyphens");
  }
  return value;
}

function luhnSum(digits) {
  let sum = 0;
  let double = false;
  for (let index = digits.length - 1; index >= 0; index -= 1) {
    let value = Number(digits[index]);
    if (double) {
      value *= 2;
      if (value > 9) value -= 9;
    }
    sum += value;
    double = !double;
  }
  return sum;
}

const sample = "4242 4242 4242 4242";
console.log(isValidCardNumber(sample));
console.log(isValidCardNumber("4242424242424241"));
console.log(cardCheckDigit("424242424242424"));