expand-shrink.js

function* shrinkGen(s) {
  let i = -1, c;
  while (c = s.charCodeAt(++i)) yield c << 0 | (s.charCodeAt(++i) || 0) << 8;
}

function shrink(input) {
  return String.fromCodePoint(...shrinkGen(input));
}

function* expandGen(str) {
  let i = -1, c;
  while (c = str.codePointAt(++i)) {
    yield c & 0x00FF;
    const high = (c & 0xFF00) >> 8;
    if (high) yield high;
  }
}

function expand(input) {
  return String.fromCharCode(...expandGen(input));
}