up_AND_down.js

const arrange = (str) => {
  let words = str.split(' '), i = 0;

  for (; i < words.length - 1; i++) {
    if (
      (0 === i % 2 && words[i].length > words[i + 1].length) ||
      (1 === i % 2 && words[i].length < words[i + 1].length)
    ) {
      let tmp = words[i];
      words[i] = words[i + 1];
      words[i + 1] = tmp;
    }
  }

  return words.map(function(word, i) {
    return i % 2 == 0 ? word.toLowerCase() : word.toUpperCase();
  }).join(' ');
};