我想从expo获取一个firebase令牌，并且必须将该令牌发送到节点js，使用我必须发送推送通知 . 每当我运行下面的代码时，它会显示未处理的promise拒绝：引用错误：找不到变量：self

import React from 'react';
import { StyleSheet, Text, View } from 'react-native';

import * as firebase from 'firebase';
import '@firebase/messaging';
export default class App extends React.Component {
   async componentDidMount(){

      const config = {

        apiKey: "xxxx",
        authDomain: "xxxx",
        databaseURL: "xxxx",
        projectId: "xxx",
        storageBucket: "xxxx",
        messagingSenderId: "xxx",
      debug: true
      };
     firebase.initializeApp(config);

firebase.messaging().getToken()
  .then(fcmToken => {
    if (fcmToken) {
      // user has a device token
      console.log("check for tokens",fcmToken)
    } else {
      console.log("err")
      // user doesn't have a device token yet
    } 
  });


}
render() {
    return (

        <Text>Open up App.js to start working on your app!</Text>

    );
  }

  }
node.js firebase expo