kaprekar.js

function kaprekar(n) {
  const asc = [].slice.call('000' + n).slice(-4).sort((a,b) => a - b).join('')|0
  const des = [].slice.call('000' + n).slice(-4).sort((a,b) => b - a).join('')|0

  return des - asc
}

function iterations(n) {
  let i = 0

  while(n != 6174) {
    n = kaprekar(n)
    i++
  }
  return i
}

function valid(n) {
  const s = '' + n
  for(let i = 1; i < s.length; i++) {
    if(s[i] != s[i-1]) { return true }
  }
  return false
}

let max = 0
for(let i = 1000; i <= 9999; i++) {
  if (valid(i)) {
    max = Math.max(max, iterations(i))
  }
}

console.log(max)