Hi Bastiaan, 

So, first of all, if you want to know more about Service Portal I would much rather look into AngularJS instead of what you have found here. 

Here is a very simple plnkr of how to move from step 1 to 2 to 3 etc. by using Angular. Just so you can understand the differences : https://plnkr.co/edit/ofha6qAyNgJZj7XQ5Zk7?p=preview&preview 

Here is one that has a funtional form, where you can see how it somehow translates. You can then compare what you have in your HTML example to the one only using angulkar: https://codepen.io/jaa2015/pen/GqparY 

The JS part is what would be the Client Script on the Service Portal. So everything after the controller (which is the client controller part on the widget). Instead of using the document and classes, you will be using $scope, ng-model, ng-class and basically just plain simple angular methods. 

Hope it makes sense and gives you something you can dig into! 🙂