App.js

import React, { Component } from 'react'
import { View, Text, TouchableOpacity } from 'react-native'
import FBSDK from 'react-native-fbsdk'

const {
  LoginButton,
  AccessToken
} = FBSDK;
 

export default class App extends Component {
    render(){
        return (
            <View style={{ flex:1, justifyContent: 'center', alignItems:'center' }}>
                <Text style={{ fontSize:25, marginBottom:15 }}>Please Log in</Text>
                <LoginButton
                    publishPermissions={["publish_actions"]}
                    onLoginFinished={(error, result) => {
                        if (error) {
                            alert("login has error: " + result.error);
                        } else if (result.isCancelled) {
                            alert("login is cancelled.");
                        } else {
                            AccessToken.getCurrentAccessToken().then((data) => {
                                alert(data.accessToken.toString())
                            })
                        }
                    }
                } 
                onLogoutFinished={() => alert("logout.")}/>
            </View>
        )
    }
}