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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.
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one pretty impressive vanilla codebase in my field is called biodalliance https://github.com/dasmoth/dalliance however, vanilla projects often fall victim to spaghetti code especially for front end UI. you have to be diligent to keep things component based. i personally think this is the biggest mind-shift that react applied to the world. it made it much more strict and structured to make components on the page.
React Repo
Angular Repo
I think that Codemirror 5 is a nice example, version 6 is somewhat hard to follow, there are way too many modules and it's hard to understand.
Yeah, I'm also not sure if he's right. Looks like the application isn't technically written with vanilla js because they use typescript. https://github.com/microsoft/vscode