auth.js

// The client id is obtained from the Google APIs Console at https://code.google.com/apis/console
// If you run access this code from a server other than http://localhost, you need to register
// your own client id.
var OAUTH2_CLIENT_ID = '__YOUR_CLIENT_ID__';
var OAUTH2_SCOPES = [
  'https://www.googleapis.com/auth/youtube'
];

// This callback is invoked by the Google APIs JS client automatically when it is loaded.
googleApiClientReady = function() {
  gapi.auth.init(function() {
    window.setTimeout(checkAuth, 1);
  });
}

// Attempt the immediate OAuth 2 client flow as soon as the page is loaded.
// If the currently logged in Google Account has previously authorized OAUTH2_CLIENT_ID, then
// it will succeed with no user intervention. Otherwise, it will fail and the user interface
// to prompt for authorization needs to be displayed.
function checkAuth() {
  handleAuthResult(true);
}

// Handles the result of a gapi.auth.authorize() call.
function handleAuthResult(authResult) {
  if (authResult) {
    // Auth was successful; hide the things related to prompting for auth and show the things
    // that should be visible after auth succeeds.
    //$('.pre-auth').hide();
    loadAPIClientInterfaces();
  }
}

// Loads the client interface for the YouTube Analytics and Data APIs.
// This is required before using the Google APIs JS client; more info is available at
// http://code.google.com/p/google-api-javascript-client/wiki/GettingStarted#Loading_the_Client
function loadAPIClientInterfaces() {
  gapi.client.load('youtube', 'v3', function() {
    handleAPILoaded();
  });
}