How to check if user is logged in or not with “Google Sign In” (OAuth 2.0)

Question

I am implementing Google log in for the first time as described here and here.

I am using HTML with Javascript.

The problem that needs solving is as follows:

Answer 1

You can stringify a custom userEntity object and store it in sessionStorage where you can check it anytime you load a new page. I have not tested the following but it should work (doing something similar with WebAPI tokens in the same way)

function onSignIn(googleUser) 
{
  var profile = googleUser.getBasicProfile();
  console.log('ID: ' + profile.getId()); 
  console.log('Name: ' + profile.getName());
  console.log('Image URL: ' + profile.getImageUrl());
  console.log('Email: ' + profile.getEmail());
  
  var myUserEntity = {};
  myUserEntity.Id = profile.getId();
  myUserEntity.Name = profile.getName();
  
  //Store the entity object in sessionStorage where it will be accessible from all pages of your site.
  sessionStorage.setItem('myUserEntity',JSON.stringify(myUserEntity));

  alert(profile.getName());   
}

function checkIfLoggedIn()
{
  if(sessionStorage.getItem('myUserEntity') == null){
    //Redirect to login page, no user entity available in sessionStorage
    window.location.href='Login.html';
  } else {
    //User already logged in
    var userEntity = {};
    userEntity = JSON.parse(sessionStorage.getItem('myUserEntity'));
    ...
    DoWhatever();
  }
}

function logout()
{
  //Don't forget to clear sessionStorage when user logs out
  sessionStorage.clear();
}

Of course, you can have some internal checks if the sessionStorage object is tampered with. This approach should work with modern browsers like Chrome and Firefox.