code: function authorize(credentials, callback) {
      var clientSecret = credentials.installed.client_secret;
      var clientId = credentials.installed.client_id;
      var redirectUrl = credentials.installed.redirect_uris[0];
      var auth = new googleAuth();
      var oauth2Client = new auth.OAuth2(clientId, clientSecret, redirectUrl);
  
      // Check if we have previously stored a token.
      fs.readFile(TOKEN_PATH, function(err, token) {
          if (err) {
              getNewToken(oauth2Client, callback);
          } else {
              oauth2Client.credentials = JSON.parse(token);
              callback(oauth2Client);
          }
      });
  }
docs: This is all from the Google Drive Quickstart 
  Create an OAuth2 client with the given credentials, and then execute the
  given callback function.
  
  @param {Object} credentials The authorization client credentials.
  @param {function} callback The callback to call with the authorized client.