It'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.
Social networks
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.