Google login for server-side apps

Question

I'm following the Google guide for signing in with Google and sending the access code to a server:

Guide here: https://developers.google.com/identity/sign-in/web/server-side-flow#step_6_send_the_authorization_code_to_the_server

I have two really simple questions:

• How does the closure in the jQuery click event know about auth2 ? If you run this code in Chrome, you'll get an error saying auth2 is undefined. How would I go about fixing that error?

Run from onload:

function start() {
    gapi.load('auth2', function() {
        auth2 = gapi.auth2.init({...});
    });
}

jQuery click handler

(function($) {
    $('#signinButton').click(function() {
        auth2.grantOfflineAccess({'redirect_uri': 'postmessage'}).then(signInCallback);
    });
})(jQuery);

• How is signInCallback passed information? Is this part of the callback process?

The code:

function signInCallback(authResult) {
    if(authResult['code']) {
        $.ajax({...});
    } else { ... }
}

Answer 1

Loading the client api at the top of the HTML file:

 <script src="https://apis.google.com/js/client:platform.js?onload=start" async defer></script>

and then initializing:

function start() {
    gapi.load('auth2', function() {
        auth2 = gapi.auth2.init({...});
    });
}

also in the header will do the trick. The guide says this, but neglects to state that it won't really work otherwise.