code: function set(key, value) {
    const entry = this.get(key, true);
    let oldvalue;
    if (entry) {
      oldvalue = entry.value;
      entry.value = value;
    } else {
      oldvalue = this.put(key, value);
      if (oldvalue) oldvalue = oldvalue.value;
    }
    return oldvalue;
  }
docs: Update the value of entry with <key>. Returns the old value, or undefined if
  entry was not in the cache.