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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.
Also if you are using classes to privatisy some methods or attributes, it will be just "private" before compilation. If you really want to use a private value you could use a closure factory, also this could reduce just a little bit the bundle size of your compiled code because there now any necesity to compile anything like when you are using a class. If you are looking for a example of that let check this rewrite that Mark Erikson did in the Subscription of react-redux.