App.js

import React, {Component} from 'react';
import {Platform, StyleSheet, Text, View} from 'react-native';
import { LoginButton, AccessToken } from 'react-native-fbsdk';

const instructions = Platform.select({
  ios: 'Press Cmd+R to reload,\n' + 'Cmd+D or shake for dev menu',
  android:
    'Double tap R on your keyboard to reload,\n' +
    'Shake or press menu button for dev menu',
});

export default class App extends Component {
  render() {
    return (
      <View style={styles.container}>
        <LoginButton
          onLoginFinished={
            (error, result) => {
              if (error) {
                console.log("login has error: " + result.error);
              } else if (result.isCancelled) {
                console.log("login is cancelled.");
              } else {
                AccessToken.getCurrentAccessToken().then(
                  (data) => {
                    console.log(data.accessToken.toString())
                  }
                )
              }
            }
          }
          onLogoutFinished={() => console.log("logout.")}/>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#F5FCFF',
  },
  welcome: {
    fontSize: 20,
    textAlign: 'center',
    margin: 10,
  },
  instructions: {
    textAlign: 'center',
    color: '#333333',
    marginBottom: 5,
  },
});