How to know if data on firebase is saved before moving to next webpage?

I am making a signup page. Users enter their name, username, email, password for sign up. After they clicked sign up they are redirected to new page, but before moving that I want to make sure their entered data is saved in firebase database. How to make sure data is saved before moving on to next webpage?

function SignUP(){    
    const emailID = _email.value;
    const pass = _password.value;
    const auth = firebase.auth();
    const promise = auth.createUserWithEmailAndPassword(emailID, pass);
    promise.catch(e => console.log(e.message));
}
function writeUserData(user, fname, lname) {
  var myuser = {
    username: name,
    firstName: fname,
    lastName: lname
  };
    firebase.database().ref('/users').push(myuser);
    window.open("chat.html",'_self',false);
}
firebase.auth().onAuthStateChanged(firebaseUser => {
   if(firebaseUser){
       console.log(firebaseUser);
       const fName = _firstName.value;
        const lName = _lastName.value;
        const uName = _username.value;
        writeUserData(uName, fName, lName);
   }else{
       console.log('not logged in');
}});