如何更新firebase文档而不覆盖现有值

{
  "name": "Alice",
  "age": 30,
  "email": "alice@example.com"
}

const firebase = require('firebase/app');
require('firebase/firestore');

// 初始化 Firebase 应用
const firebaseConfig = {
  // 你的 Firebase 配置
};

firebase.initializeApp(firebaseConfig);
const db = firebase.firestore();

// 更新文档中的 age 字段
db.collection('users').doc('user1').update({
  age: 31
})
.then(() => {
  console.log("Document successfully updated!");
})
.catch((error) => {
  console.error("Error updating document: ", error);
});

db.collection('users').doc('user1').get()
.then(doc => {
  if (!doc.exists()) {
    console.log("No such document!");
  } else {
    return doc.ref.update({ age: 31 });
  }
})
.then(() => {
  console.log("Document successfully updated!");
})
.catch((error) => {
  console.error("Error updating document: ", error);
});