reverseonlyletter.js

function isLetter(codePoint) {
  if (65 <= codePoint && codePoint <= 90) return true;
  if (97 <= codePoint && codePoint < 123) return true;
  // maybe more cases for unicode
  return false;
}

function reverseOnlyLetters(string) {
  if (typeof string !== "string") return;
  const length = string.length;
  const array = length < 50 ? [] : Array(length);
  for (let i = 0, k = 0; ; ++i, ++k) {
    const c = string.codePointAt(i);
    array[k] = c;
    if (c > 0xFFFF) ++i;
    if (i >= length) {
      if (array.length !== k) array.length = k;
      break;
    }
  }
  for (let start = 0, end = array.length - 1; ; ++start, --end) {
    while (!isLetter(array[start])) ++start;
    while (!isLetter(array[end])) --end;
    if (start >= end) break;
    const temp = array[start];
    array[start] = array[end];
    array[end] = temp;
  }
  return String.fromCodePoint(...array);
}