WindowOnMessage.js

...

window.onMessage = function(event) {
  // now you'll follow with your callback
}

facebookLoginClicked() {
  // how to post a message
  switch(window.isNative) {
    case 'android':
      window.postMessage('facebookLogin')
      break

    case 'ios':
      window.webkit.messageHandlers.reactNative.postMessage('facebookLogin')
      break

    default:
      // not native
      window.facebookLogin()
  }
}