firebase check signed in code example

Example 1: restrict the user from going to signup or login page if the user is already logged in firebase auth

var user = firebase.auth().currentUser;

if (user) {
  // User is signed in.
} else {
  // No user is signed in.
}

Example 2: google auth.onstatechange

firebase.auth().onAuthStateChanged(function(user) {  if (user) { 
// User is signed in. 
} 
  else {   
    // No user is signed in.
  }});