2m readIt's really great to see the developer community growing to this milestone, it's a great place to meet other members, exchange ideas and new concepts. Congratulations to the whole DC team, keep the community growing! On to the upcoming next 5 years! 
Front-end technology changes much faster than back-end technology. As Dmitry also suggests, for best results I can recommend you for writing web based apps to use modern front-end frameworks like Vue.js/NuxtJS, React, Angular, Svelte. These frameworks allow you to write well-structured and maintainable code, separating front-end nicely from your back-end. See also this NuxtJS tutorial article. Btw, if you're most used to HTML/CSS/JS in your pages, Vue.js will feel the most familiar to work with and it allows you too to update your existing pages gradually by including the Vue.js script in your <head> tag and start using it for parts of your existing HTML code.
To interact with you back-end IRIS/Caché server, you have two major options: using the REST application server built into IRIS/Caché (if you want to write your back-end completely in ObjectScript) or use a Node.js applications server built on the very popular Express npm module: QEWD-Up. This Node.js back-end server is both a REST & WebSockets application server. It allows you to write JavaScript both in your front-end and back-end code.
In addition, the Node.js option has the advantage you can use all existing npm modules too: for nearly every feature you can imagine, you'll find an existing npm module - you don't need to re-invent the wheel! E.g. you need Google maps in your app? There's a module for that!
WebSockets in contrast to REST give you a very efficient open connection to your IRIS/Caché back-end without the overhead of a REST call. And the most important part: QEWD-Up takes care of all setup and boilerplate code so you can focus on your application code.
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Very nice integration PoC, showing the power of using the Node.js ecosystem. Combining mainstream Node.js tools & modules with IRIS allows you to create very powerful applications without having to reinvent the wheel.