Firebase上的Javascript:once（“value”）中出现错误。然后（）；

为了在Firebase上设置一些云函数，我在两天前开始学习Javascript，尝试在“AchievementCount_total”节点下计算和存储一些数据（请参考图了解数据库结构）。我已经成功地创建了节点。然而，存储在该节点下的数据并不是我所期望的。为了测试发生了什么，我修改了代码，如下所示：

const functions = require('firebase-functions');
// The Firebase Admin SDK to access the Firebase Realtime Database. 
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.updateScores = functions.database.ref('/users/{userUID}/achievements')
.onWrite(event => {
  var database=admin.database();
  var achievementsCount_total=new Array();//not important here
  for(var i=0;i<60;i++){achievementsCount_total[i]=0;}//not important here
  var allUsersRef=database.ref("/users");//a list containing all users
  var hasChildren=123;
  allUsersRef.once("value").then(function (snapshot)//SOMETHING WENT WRONG HERE!
  {
    hasChildren=snapshot.hasChildren();//either "true" or "false"
    snapshot.forEach(function(element) 
    {
      // . . . not important here
    });
  });
  return admin.database().ref("/achievementsCount_total").set(hasChildren);      
});

看起来函数甚至没有执行（因此变量“haschilds”的值仍然是123），我不知道为什么。有人知道答案吗？多谢各位~

allUsersRef.once("value").then(function (snapshot)//SOMETHING WENT WRONG HERE!
  {
    hasChildren=snapshot.hasChildren();//either "true" or "false"
    snapshot.forEach(function(element) 
    {
      // . . . not important here
    });
  });

是一个异步函数

return admin.database().ref("/achievementsCount_total").set(hasChildren);

不等待它运行，为了修复此问题，您必须在中返回它。然后（）
这应该行得通

  return allUsersRef.once("value").then(function (snapshot)//SOMETHING WENT WRONG HERE!
  {
    hasChildren=snapshot.hasChildren();//either "true" or "false"
    snapshot.forEach(function(element) 
    {
      // . . . not important here
    });
    return admin.database().ref("/achievementsCount_total").set(hasChildren);
  });

  return allUsersRef.once("value").then(function (snapshot)//SOMETHING WENT WRONG HERE!
  {
    hasChildren=snapshot.hasChildren();//either "true" or "false"
    snapshot.forEach(function(element) 
    {
      // . . . not important here
    });
    return admin.database().ref("/achievementsCount_total").set(hasChildren);
  });