问如何将Google API作为特定用户的服务器端使用？Node.js

EN

var google = require( 'googleapis' );
var gmail = google.gmail('v1');

var key = require('../client_secret.json');

var authClient = new google.auth.JWT(
    key.client_email,
    null,
    key.private_key,
    ['https://mail.google.com/'],
    '****@gmail.com');

authClient.authorize(function (err, tokens) {
  if (err) {
    return console.log(err);
  }

  gmail.users.labels.list({
    auth: authClient,
    userId: '****@gmail.com'
  }, function (err, resp) {
    if (err) {
      return console.log(err);
    }

    console.log(resp);
  });
});

[Error: unauthorized_client]