index.js

firebase.auth().onAuthStateChanged(function(user) {
  if (user) {
    // User is signed in.

    document.getElementById("user_div").style.display = "block";
    document.getElementById("login_div").style.display = "none";

    var user = firebase.auth().currentUser;

    if(user != null){

      var email_id = user.email;
      document.getElementById("user_para").innerHTML = "Welcome User : " + email_id;

    }

  } else {
    // No user is signed in.

    document.getElementById("user_div").style.display = "none";
    document.getElementById("login_div").style.display = "block";

  }
});

function updateMeals() {

  var breakfastValue = document.getElementById("breakfast_textField").value;
  var teaValue = document.getElementById("tea_textField").value;
  var lunchValue = document.getElementById("lunch_textField").value;
  var dinnerValue = document.getElementById("dinner_textField").value;
  var dateValue = document.getElementById("date_textField").value;

window.alert(dateValue)
  firebase.database().ref("Stuff/"+dateValue).set({
    "breakfast": breakfastValue,
    "tea": teaValue,
    "lunch": lunchValue,
    "dinner": dinnerValue
  });

}

function login(){

  var userEmail = document.getElementById("email_field").value;
  var userPass = document.getElementById("password_field").value;

  firebase.auth().signInWithEmailAndPassword(userEmail, userPass).catch(function(error) {
    // Handle Errors here.
    var errorCode = error.code;
    var errorMessage = error.message;

    window.alert("Error : " + errorMessage);

    // ...
  });

}

function logout(){
  firebase.auth().signOut();
}