code: function get(key, returnEntry) {
    // First, find our cache entry
    const entry = this._keymap[key];
    if (entry === undefined) return undefined; // Not cached. Sorry.
    // As <key> was found in the cache, register it as being requested recently
    if (entry === this.tail) {
      // Already the most recenlty used entry, so no need to update the list
      return (returnEntry ? entry : entry.value);
    }
    // HEAD--------------TAIL
    //   <.older   .newer>
    //  <--- add direction --
    //   A  B  C  <D>  E
    if (entry.newer) {
      if (entry === this.head) this.head = entry.newer;
      entry.newer.older = entry.older; // C <-- E.
    }
    if (entry.older) entry.older.newer = entry.newer; // C. --> E
    entry.newer = undefined; // D --x
    entry.older = this.tail; // D. --> E
    if (this.tail) this.tail.newer = entry; // E. <-- D
    this.tail = entry;
  
    return (returnEntry ? entry : entry.value);
  }
docs: Get and register recent use of <key>. Returns the value associated with <key>
  or undefined if not in cache.