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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.
On a larger scale for your applications, you will probably make use of more optional fields when it comes to creating your user's information. However, once you are able to handle the user's log in, the sky is the limit to what you can do with their data. This will work at a basic level with just an email and password, but if you wish to read more about json-server-auth and some of it's further capabilities with tokens or authorization flow you can find its documentation here.