app.js

(function(d, s, id){
var js, fjs = d.getElementsByTagName(s)[0]
if (d.getElementById(id)) {return}
js = d.createElement(s); js.id = id
js.src = "//connect.facebook.net/en_US/sdk.js"
fjs.parentNode.insertBefore(js, fjs)
}(document, 'script', 'facebook-jssdk'))

window.fbAsyncInit = function() {
  FB.init({
    appId: 'APPID',
    status: true,
    xfbml: true,
    version: 'v2.9'
  })
}

// Receive message from webview
window.onMessage = function(data) {
  // finish facebook authentication flow
}

// Web Facebook Login
function loginFacebook(response) {
  if(response && response.status !== 'connected') {
    return alert('denied')
  } else if(response && response.status === 'connected') {
    return alert('logged in')
  }

  FB.getLoginStatus(function(response) {
    if(response.status !== 'connected') {
      FB.login(loginFacebook)
    } else {
      alert('logged in')
    }
  })
}

function triggerLogin(platform) {
  switch(window.isNative) {
    case 'android':
      window.postMessage(platform)
      break

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

    default:
      window[platform]()
  }
}