calculate-persistence.js

/*
 * Calculate the persistence of a number;
 * based on Numberphile video:
 * https://www.youtube.com/watch?v=Wim9WJeDTHQ
 */
function calculatePersistence (num) {
  let result = 1
  while ((num = multiplyItems(getDigits(num))) > 0)
    result++
  return result
}

function getDigits (num) {
  const result = []

  while (num > 0) {
    const rem = num % 10

    result.push(rem)
    num -= rem
    num /= 10
  }

  return result
}

function multiplyItems (items) {
  return items.reduce((result, item) =>
    (result * item),
  1)
}