is
Whether the method can be called from JS synchronously on the JS thread, possibly returning a result.
WARNING: in the vast majority of cases, you should leave this to false which allows your native module methods to be called asynchronously: calling methods synchronously can have strong performance penalties and introduce threading-related bugs to your native modules.
In order to support remote debugging, both the method args and return type must be serializable to JSON: this means that we only support the same args as ReactMethod, and the hook can only be void or return JSON values (e.g. bool, number, String, [ ], or WritableArray). Calling these methods when running under the websocket executor is currently not supported.