auth.js

var auth2;
var loggedInPara = document.getElementById('login-status');

gapi.load('auth2', function(){
    auth2 = gapi.auth2.init({
        client_id: 'app-id.apps.googleusercontent.com'
    });
    loggedInPara.textContent =  (auth2.isSignedIn.get()) ? 'User is logged in' : 'User is logged out';
    auth2.attachClickHandler('signin-button', {}, onSuccess, onFailure);
});  

var onSuccess = function(user) {
    console.log('Signed in as ' + user.getBasicProfile().getName());
};

var onFailure = function(error) {
    console.log(error);
};

function signOut() {
    auth2.signOut().then(function () {
        console.log('User signed out.');
    });
}        

function isLoggedIn(){
    return auth2.isSignedIn.get();
}