using EFFC.ChakraCore;
using EFFC.VRoomJs;
 ...
 JsEngineSwitcher engineSwitcher = JsEngineSwitcher.Instance;
 engineSwitcher.EngineFactories
                .AddEFFCChakraCore()
               .AddEFFCVroom(new EFFCVroomSettings
               {
                  MaxYoungSpaceSize = 4194304,
                  MaxOldSpaceSize = 8388608
             });
switch (name.ToLower())
           {
               case "chakra":
                  engineSwitcher.DefaultEngineName = EFFCChakraCoreJsEngine.EngineName;
                  break;
               case "vroomjs":
                  engineSwitcher.DefaultEngineName = EFFCVroomJsEngine.EngineName;
                    break;
              default:
                   engineSwitcher.DefaultEngineName = EFFCChakraCoreJsEngine.EngineName;
                   break;
          }
          js = engineSwitcher.CreateDefaultEngine();