问在firebase (javascript)中，如何获取当前登录用户的数据？

EN

let ref = database.ref('/users/' + currentUser.uid).once('value').then(function(snapshot) {
    let userData = snapshot.val();
    console.log(userData.country);
}

function showUser(){
   var usref=firebase.database().ref("countries/"+uid);
   //uid here is the unique code u gave to the specif user
   usref.once('value', function(snapshot) {snapshot.forEach(function(childSnapshot) {
   if((childSnapshot.key)=="country"){
     var country = childSnapshot.val();
     //use this variable here enter code here
     window.alert(country);//just for checking!!!
   }
   if((childSnapshot.key)=="age"){
     var name = childSnapshot.val();
     //use this variable here
     window.alert(name);//just for checking!!!
   }
   });
 });
}