换个思路

提高了速度，但是也增加了代码量，分享一个思路吧。

还可以吧通用的获取字母码点的值提取出来

代码

const stringSort = (str) => {
  const arr = new Array(62).fill(0);
  for (let i = 0; i < str.length; i++) {
    const e = str[i];
    if (/\d/.test(e)) {
      arr[Number(e)] += 1;
    } else if (/[A-Z]/.test(e)) {
      arr[10 + e.codePointAt() - 'A'.codePointAt()] += 1;
    } else {
      arr[36 + e.codePointAt() - 'a'.codePointAt()] += 1;
    }
  }
  return arr.reduce((init, cur, idx) => {
    return init + getCharsFromIdx(idx, cur);
  }, '');
};

const getCharsFromIdx = (idx, repeat) => {
  if (idx < 10) {
    return String(idx).repeat(repeat);
  } else if (idx < 36) {
    return String.fromCodePoint(idx - 10 + 'A'.codePointAt()).repeat(repeat);
  } else {
    return String.fromCodePoint(idx - 36 + 'a'.codePointAt()).repeat(repeat);
  }
};

while(input = readline()) {
  console.log(stringSort(input))
}