script.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";
  } else {
    // No user is signed in.
    document.getElementById("user_div").style.display = "none";
    document.getElementById("login_div").style.display = "block";
  }
});

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()
  }