function garbageCollect(caches, length) {
  each(caches, (cache) => {
    const gc = cache.gc;
    const gcLen = gc.length / 2;
    let i;
    if (gcLen > length) {
      for (i = 0; i < gcLen; ++i) {
        delete cache.data[gc[i]];
      }
      gc.splice(0, gcLen);
    }
  });
}
@param {object} caches
@param {number} length