- Javasctipt DOM Elements - https://www.w3schools.com/js/js_htmld...
- KeydownEvent - Add category functionality - change event, prevent default
- Keydown Event Handler - https://www.w3schools.com/jsref/event...
- Key Code Info - http://keycode.info/
- Let vs Const by Wes Bos - http://wesbos.com/let-vs-const/