Firebase在子循环中检索单个值

var userId = "user1";
firebase.database().ref("users/" + userId).once("value")
  .then(function(snapshot) {
    var user = snapshot.val();
    console.log(user);
  });

var userId = "user1";
firebase.firestore().collection("users").doc(userId).get()
  .then(function(doc) {
    if (doc.exists) {
      var user = doc.data();
      console.log(user);
    } else {
      console.log("User not found");
    }
  });