Passing swift block to React Native

Is it possible to pass block from Swift to React Native as prop? I'm trying to do:

    let rnRoot = RCTRootView(
        bundleURL: URL(string: "http://localhost:8081/index.ios.bundle?platform=ios"),
        moduleName: "App",
        initialProperties: ["callLog": { (arg: String) in
        launchOptions: nil

It seems to me, that callLog gets passed and even RN gets the idea, that it's function, logs are:

2018-01-14 12:28:11.650130+0100 prj[46427:1071345] Running application App ({
    initialProps =     {
        callLog = "(Function)";
    rootTag = 1;

But it starts with callLog as null anyway:

Running application "App" with appParams: {"rootTag":1,"initialProps":{"callLog":null}}. DEV === true, development-level warning are ON, performance optimizations are OFF

Is it possible to do that at all?