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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
The README is a lot more descriptive with code samples and the general concepts of the library nicely illustrated: https://github.com/sebringrose/peko
Regarding the build step, Peko doesn't (yet?) have a handler directly for this but I have implemented this kind of logic in the iiisun.art app here. The code is still a WIP but basically a loading screen is sent at first which triggers compilation assets, then the "/" is swapped to the index page and an event is sent to the client telling it to refresh. There might be a cool way to wrap this behaviour but part of the mantra of Peko has been to keep it lightweight and flexible so would need to find a good solution. If you have suggestions I'd love to hear them :)