index2.js

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

    document.getElementById("user_div").style.display = "block";
    document.getElementById("signup_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("signup_div").style.display = "block";

  }
});

function signup(){

  var email = document.getElementById("email_field").value;
  var password = document.getElementById("password_field").value;
 
  // Email sent.

firebase.auth().createUserWithEmailAndPassword(email, password).catch(function(error) {
  // Handle Errors here.
  var errorCode = error.code;
  var errorMessage = error.message;

  // ...


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

    // ...
  });

}

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