data structure that always preserves the previous version of itself when it is modified

Imagine web apps with fast browser load times that also offer amazing developer productivity and require less code to create. That's what Svelte and Sapper deliver! Svelte pushes a lot of the work a frontend framework would handle to the compile step, so your app components come out as tight, well-organized JavaScript modules. Sapper is a lightweight web framework that minimizes application size through server-rendering front pages and only loading the JavaScript you need. The end result is more efficient apps with great UX and simplified state management.