CandyCrushish.js

const crushOnce = (xs) => {
  if (xs.length < 3) return xs;
  let count = 0;
  
  for (const x of xs) {
    if (x !== xs[0]) break;
    count++;
  }

  if (count >= 3) {
    return crush(xs.slice(count));
  }

  return xs.slice(0, count) + crush(xs.slice(count));
};

const crush = (ss) => {
  const crushed = crushOnce(ss);
  if(crushed == ss) return ss;
  return crushOnce(crushed);
}

crush('aabbccddeeedcbak'); // "k"