$ npm install youthumb
// http://img.youtube.com/vi/I9khEhPNbOc/mqdefault.jpg
youthumb('I9khEhPNbOc').get('medium');
// http://img.youtube.com/vi/I9khEhPNbOc/hqdefault.jpg
youthumb('I9khEhPNbOc').get('high');
// http://img.youtube.com/vi/I9khEhPNbOc/1.jpg
youthumb('I9khEhPNbOc').get(1);
// http://img.youtube.com/vi/I9khEhPNbOc/default.jpg
youthumb('I9khEhPNbOc').get();
youthumb('I9khEhPNbOc').get('default');
youthumb('I9khEhPNbOc').get(100);
youthumb('I9khEhPNbOc').get('not exists');