每个then（）应该返回一个值或引发通知

Question

我是javascript新手，每个then（）都出错。我想在数据库发生更改时发送通知。

index.js

'use strict'

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.sendNotification = functions.firestore.document("Buyers/{buyerID}/Notifications/{notificationId}").onWrite(event => {
    const buyerID = event.params.buyerID;
    const notificationId = event.params.notificationId;

    return admin.firestore().collection("Buyers").doc(buyerID).collection("Notifications").doc(notificationId).get().then(queryResult => {
        const notificationMessage = queryResult.data().notificationMessage;
        const notificationTitle = queryResult.data().notificationTitle;

        return Promise.all([fromUser, toUser]).then(result => {
            const tokenId = result[0].data().tokenId;

            const notificationContent = {
                notification: {
                    title: notificationTitle,
                    body: notificationMessage,
                    icon: "default",
                    sound : "default"
                } 
            };

            return admin.messaging().sendToDevice(tokenId, notificationContent).then(result => {
                console.log("Notification sent!");
            });
        });
    })
});