Firebase查询if语句

Question

我正在尝试执行Firestore查询，以检查数据库中是否已存在电子邮件。我想用一个简单的函数来执行此操作，而不是检查函数是否返回false或true，但是if语句在函数被解雇之前给出了输出，有人可以帮助我吗？

const admin     =   require("firebase-admin");

// Initialize Firebase connection
var serviceAccount = require("./fbAccount.json");

admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: "https://ancobook-1aceb.firebaseio.com"
});

let db = admin.firestore();

function checkEmail(test) {
    let citiesRef = db.collection('users');
    let query = citiesRef.where('username', '=', test).get()
      .then(snapshot => {
        if (snapshot.empty) {
            console.log('No matching documents.');
          return true;
        }  

        snapshot.forEach(doc => {
            console.log('Matching documents');
          return false;
        });
      })
}

if (checkEmail('brancoschoenaker@gmail.com') == false) {
    console.log('Account already exists')
} else {
    console.log('Created Account')
}