This article was originally written quite a while ago, when the transition from AngularJS to Angular was just around the corner as well as Ionic 2. Quite some time has passed since then and the modern versions of both Angular and Ionic are used heavily today.
At that time “modern” JavaScript using ECMAScript 6 was not as commonly well known or as used as it is today. However, having an understanding of what ECMAScript 6 is and the new features it brings to JavaScript is still relevant today and will help you build applications.
I have updated and rewritten parts of this article to be more relevant to the current versions of Ionic and Angular.