check if a user is already logged in with Firebase in Browser

Question

Is there any way to check user logged in with firebase in Browser. I want to check in broswer if user logged in or not?

<script src="https://www.gstatic.com/firebasejs/4.12.1/firebase.js"></script>
<script src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<script>
var config = {  
apiKey: "Apikey",
authDomain: "WebAppDomain",
databaseURL: "URL",
projectId: "ProjectId",
storageBucket: "",
messagingSenderId: ""
};
firebase.initializeApp(config);
$( document ).ready(function() {
    console.log(firebase.auth()) 
    console.log( "testing.." );
    var user = firebase.auth().currentUser;
    console.log(user);
});
</script>

Answer 1

You need to use onAuthStateChanged.

firebase.auth().onAuthStateChanged((user) => {
    if (user) {
        // user exists, do stuff
    } else {
        // no user
    }
});