如何修复ReferenceError:当所有SDK都在HTML文件中时，不定义firebase

<!-- Firebase App (the core Firebase SDK) is always required and must be listed first -->
<script src="https://www.gstatic.com/firebasejs/9.6.1/firebase-app.js"></script>

<!-- If you enabled Analytics in your project, add the Firebase SDK for Analytics -->
<script src="https://www.gstatic.com/firebasejs/9.6.1/firebase-analytics.js"></script>

<!-- Add other Firebase SDKs that you want to use -->
<script src="https://www.gstatic.com/firebasejs/9.6.1/firebase-auth.js"></script>
<script src="https://www.gstatic.com/firebasejs/9.6.1/firebase-firestore.js"></script>

// Your web app's Firebase configuration
const firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "YOUR_AUTH_DOMAIN",
  projectId: "YOUR_PROJECT_ID",
  storageBucket: "YOUR_STORAGE_BUCKET",
  messagingSenderId: "YOUR_MESSAGING_SENDER_ID",
  appId: "YOUR_APP_ID"
};

// Initialize Firebase
firebase.initializeApp(firebaseConfig);

<script src="https://www.gstatic.com/firebasejs/9.6.1/firebase-app.js"></script>
<script src="https://www.gstatic.com/firebasejs/9.6.1/firebase-auth.js"></script>
<script>
  // Firebase 初始化代码
  const firebaseConfig = {...};
  firebase.initializeApp(firebaseConfig);

  // 使用 Firebase 的代码
  firebase.auth().onAuthStateChanged(user => {
    if (user) {
      console.log('User is signed in:', user);
    } else {
      console.log('No user is signed in.');
    }
  });
</script>

<script>
  function loadFirebase() {
    return new Promise((resolve, reject) => {
      const script = document.createElement('script');
      script.src = 'https://www.gstatic.com/firebasejs/9.6.1/firebase-app.js';
      script.onload = () => {
        // Firebase SDK 加载完成后的操作
        resolve(firebase);
      };
      script.onerror = reject;
      document.head.appendChild(script);
    });
  }

  loadFirebase().then(firebase => {
    // 初始化 Firebase 并使用它
    const firebaseConfig = {...};
    firebase.initializeApp(firebaseConfig);
    // ... 使用 Firebase 的代码 ...
  }).catch(error => {
    console.error('Error loading Firebase SDK:', error);
  });
</script>