如何从firestore中获取单个文档？

import firebase from 'firebase/app';
import 'firebase/firestore';

firebase.initializeApp({
  // 替换为你的Firebase配置
  apiKey: "API_KEY",
  authDomain: "PROJECT_ID.firebaseapp.com",
  databaseURL: "https://PROJECT_ID.firebaseio.com",
  projectId: "PROJECT_ID",
  storageBucket: "PROJECT_ID.appspot.com",
  messagingSenderId: "SENDER_ID",
});

const db = firebase.firestore();

const docRef = db.collection('collectionName').doc('documentId');
docRef.get().then((doc) => {
  if (doc.exists) {
    // 文档存在，获取文档数据
    const data = doc.data();
    console.log(data);
  } else {
    // 文档不存在
    console.log('文档不存在');
  }
}).catch((error) => {
  console.log('获取文档失败', error);
});