JavaScript
In my opinion, JavaScript is the language you need to know. Yes, it has its weird parts, but you can do pretty much everything you think of with it and a bit of front-end knowledge (HTML, CSS). I use JSON even for non-JavaScript related projects. My first experience using JavaScript was mostly with jQuery as a DOM manipulator and AJAX but now I'm using it for almost everything, depending on the project's requirements. I've extensively used Node.js (Express, Socket.io, etc), Vue.js (Nuxt, Quasar Framework, etc) and sometimes just plain JavaScript using some cool new ECMAScript features. TypeScript seems great from what I've seen and I intend to dive deeper into it at a later stage.