The Luhn Algorithm in JavaScript

luhn.js

/**
 * @param {number[]} digits
 */
function luhn(digits) {
  digits = digits.reverse()
  for (const index in digits) {
    if (index % 2) {
      const doubled = digits[index] * 2
      digits[index] = doubled > 9 ? doubled - 9 : doubled
    }
  }
  const sum = digits.reduce((t, c) => t + c, 0)
  return !(sum % 10)
}
// https://en.wikipedia.org/wiki/Luhn_algorithm