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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.
You mean you want to see the source code? We do have such servers built with our framework. There you have guidelines on how to do things and here you've got a boilerplate. I cannot share the source code of a real production app with you unfortunately, but if you have specific questions (how to protect against nested deep attacks, etc) I can gladly answer you.
I don't know how well its maintained but Artsy Metaphysics is a open source graphql server and is actually used in production.