如何检查firebase数据库中是否存在值

<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>

const 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);

const database = firebase.database();
const ref = database.ref("path/to/your/data");

ref.once("value")
  .then((snapshot) => {
    const data = snapshot.val();
    if (data !== null) {
      console.log("值存在");
    } else {
      console.log("值不存在");
    }
  })
  .catch((error) => {
    console.error("读取数据时出错：", error);
  });