Firebase实时数据库快照不返回正确或错误

Question

The snapshot function I am using will not give me a value which will then not execute the code.

The database is all linked up and removing the if statement and snapshot check writes to the database. However, I want this to work as I do not want to reset the user variables so I need to check if it already exists.

function createUserData(){
    var ref = firebase.database().ref("users/" + uid);
ref.once("value")
  .then(function(snapshot) {
        userExistCheck = snapshot.exists();
    });

    if (userExistCheck == false){
        displayName = document.getElementById("name-input").value;
        firebase.database().ref('users/' + uid).set({
        username: displayName,
        email: email,
        module1: false,
        module2: false,
        module3: false,
        module4: false,
        module5: false,
        module6: false
      });
        window.alert("working")
    }
    else{
        window.alert(uid);
    }
}

If the user doesn't exist it should add the user however it doesn't. There is no output at all and the window alert for uid displays.

Answer 1

This block

ref.once("value")
  .then(function(snapshot) {
        userExistCheck = snapshot.exists();
    });

is Asynchronous, so before it retrieves your data is executing this one

 if (userExistCheck == false){
        displayName = document.getElementById("name-input").value;
        firebase.database().ref('users/' + uid).set({
        username: displayName,
        email: email,
        module1: false,
        module2: false,
        module3: false,
        module4: false,
        module5: false,
        module6: false
      });
        window.alert("working")
    }
    else{
        window.alert(uid);
    }

which will not update after the value is checked if exists or not, to solve this move your code

function createUserData(){
    var ref = firebase.database().ref("users/" + uid);
ref.once("value")
  .then(function(snapshot) {
        userExistCheck = snapshot.exists();
 if (userExistCheck == false){
        displayName = document.getElementById("name-input").value;
        firebase.database().ref('users/' + uid).set({
        username: displayName,
        email: email,
        module1: false,
        module2: false,
        module3: false,
        module4: false,
        module5: false,
        module6: false
      });
        window.alert("working")
    }
    else{
        window.alert(uid);
    }
    });


}