code: function getCacheCtor(cache){
    if (!cache){
      return;
    }
  
    if (typeof cache === 'function'){
      return cache;
    }
  
    function runtimeLoad(moduleRef){
      return require(moduleRef);
    }
  
    var cacheCtors = {
      disabled: runtimeLoad('./DisabledCache'),
      memory: runtimeLoad('./MemoryStore'),
      memcached: runtimeLoad('./MemcachedStore'),
      redis: runtimeLoad('./RedisStore')
    };
  
    if (!(cache in cacheCtors)){
      throw new Error('Unsupported cache provider: ' + cache);
    }
  
    return cacheCtors[cache];
  }
docs: private function