NewPostMessageMethod.js

...

handleNavigationStateChange({ url }) {
  ...
  
  if(Platform.OS === 'ios') {
    this.refs[webviewRef].evaluateJavaScript('var isNative = "ios"')
  } else {
    this.refs[webviewRef].injectJavaScript('var isNative = "android"')
  }
  
  ...
}
  
postMessage(message) {
  message = JSON.stringify(message)
  console.log('posting message', message)

  const finalMessage = 'window.onMessage(' + message + ')'

  if(Platform.OS === 'ios') {
    this.refs[webviewRef].evaluateJavaScript(finalMessage)
  } else {
    this.refs[webviewRef].injectJavaScript(finalMessage)
  }

  console.log('message posted')
}
  
facebookLoginMethod() {
  LoginManager
    .logInWithReadPermissions(['public_profile'])
    .then(response => {
      // send the response back to the webview
      this.postMessage(JSON.stringify({ ...response, type: 'facebookLogin' }))
    })
    .catch(console.error)
}

...