Number padder

naive.js

function padding(num, sep) {
var numarr = num['toString']().split('')
var numarrLen = numarr.length
var out = []
for (;numarrLen--;) {
  var num = numarr[numarrLen]
  out.unshift(num)
  if (numarr.length !== numarrLen && num % 3 === 0) out.unshift(sep)
}
return out.join('')
}

tricky.js

function padding(num, sep) {
  var numarr = num['toString']().split('').reverse()
  var haho = numarr.reduce((acc, el, ix) => acc.concat([ix&&!(ix % 3) ? sep : "", el]), [])
  return haho.filter(el => !!el).reverse().join('')
}