如何使用firebase适配器从单个子级获取键和值

<script src="https://www.gstatic.com/firebasejs/9.0.2/firebase-app.js"></script>
<script src="https://www.gstatic.com/firebasejs/9.0.2/firebase-database.js"></script>

var firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "YOUR_AUTH_DOMAIN",
  databaseURL: "YOUR_DATABASE_URL",
  projectId: "YOUR_PROJECT_ID",
  storageBucket: "YOUR_STORAGE_BUCKET",
  messagingSenderId: "YOUR_MESSAGING_SENDER_ID",
  appId: "YOUR_APP_ID"
};

firebase.initializeApp(firebaseConfig);

var database = firebase.database();
var usersRef = database.ref('users');

usersRef.once('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var key = childSnapshot.key;
    var value = childSnapshot.val();
    console.log("Key: " + key + ", Value: " + value);
  });
});