sep_28.js

function decipherThis(string) {
  const words = string.split(' ');
  return words.map(word => decode(word.split(''))).join(' ');
}
function decode(wordArr) {
  let hex = ''
  let secChar
  for (let [idx, char] of wordArr.entries()) {

    if (Number(char) || char == 0) {
      hex += char
    } else {
      secChar = idx
      break;
    }
  }
  let res = String.fromCharCode(hex)
  res += wordArr.length > hex.length ? wordArr.pop() : ''
  res += wordArr.length ? wordArr.splice(secChar + 1).join('') : ''
  res += wordArr[secChar] ? wordArr[secChar] : ''
  return res
}

function backSpace(string) {
  const resStack = []
  for (const char of string) {
    if (char === '#') resStack.length && resStack.pop()
    else resStack.push(char)
  }
  return resStack.join('')
}
console.log(backSpace("abc#d##c"))//      ==>  "ac"
console.log(backSpace("abc##d######"))//      ==>  "ac"
console.log(backSpace("#######"))//      ==>  "ac"
console.log(backSpace(""))//      ==>  "ac"
// "abc##d######"  ==>  ""
// "#######"       ==>  ""
// ""              ==>  ""