Vue.js
Vue.js is an awesome frontend JavaScript/Typescript framework for making all kinds of interactive, reactive, web applications. One of the coolest things about it is that you can include just a snippet of JavaScript on a page and progressively enhance the page with interactive functionality, without commiting to the whole framework. On the other hand, you can easily write full fledged SPAs like I've done with WP 1.0 and Rainfall.
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