Firebase函數(shù)：無(wú)法讀取未定義的屬性“user_id”我試圖用我的移動(dòng)應(yīng)用程序做一個(gè)簡(jiǎn)單的hello world firebase功能，我想記錄用戶ID，這樣我就可以看到該功能確實(shí)有效。這是我目前的javascript代碼：const functions = require('firebase-functions');const admin = require('firebase-admin');admin.initializeApp(functions.config().firebase);exports.sendNotification = functions.database.ref('/notifications/{user_id}').onWrite((event) => {   console.log('Testing stuff', event.params.user_id);   return;});當(dāng)新數(shù)據(jù)寫(xiě)入特定數(shù)據(jù)庫(kù)表時(shí)會(huì)觸發(fā)，但會(huì)顯示以下錯(cuò)誤：TypeError: Cannot read property 'user_id' of undefined     at exports.sendNotification.functions.database.ref.onWrite (/user_code/index.js:8:44)     at Object.<anonymous> (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:112:27)     at next (native)     at /user_code/node_modules/firebase-functions/lib/cloud-functions.js:28:71     at __awaiter (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:24:12)     at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:82:36)     at /var/tmp/worker/worker.js:700:26     at process._tickDomainCallback (internal/process/next_tick.js:135:7)