unable to obtain access_token Google+ signin using

Question

I am trying to integrate google+ login with my website using javascript sdk, Though I am able to validate the user I need to obtain an access token on successful signin which I pass to the server to obtain further information. I went through the documentation in google developer console in this link: Google Sign-In for Websites

in that is could see a method GoogleUser.getAuthResponse() whose response object must contain an access_token.

However in my response object I couldn't find the access_token , but all the remaining key were present. I am not sure if the documentation is outdated.

Please help with me a solution this problem.