var up = function(current, goal) { // add types
  console.log(current) // printf
  if(current.length === goal) {
    down(current.slice(0, -1), 0)
  } else {
    up(current + '0', goal)
  }
}

var down = function(current, goal) { // add types
  console.log(current)
  if(current.length !== goal) { // check how measure length in your language
    down(current.slice(0, -1), goal)
  }
}

up('0', 10)