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Closest I can think of is ttypescript
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Civic Auth
Web2 & Web3 login in a simple SDK. Drop Civic Auth into your app with native TS/JS support. Email login, SSO options, embedded wallets, and full session management. Minimal config. Deploy in under 5 minutes.
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As you mentioned, there are proposals.
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typescript-transformer-handbook
📘 A comprehensive handbook on how to create transformers for TypeScript with code examples.
I would start by researching TypeScript transforms and ts-morph.
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There's a weird thing called SweetJS which can be used to extend the language by way of macros.
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There are a couple of ways. Like you mentioned Babel has various features that count as language extensions and there's also SweetJS. I've been interested in exploring the compiler API personally and ts-morph.
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SurveyJS
JavaScript Form Builder with No-Code UI & Built-In JSON Schema Editor. Add the SurveyJS white-label form builder to your JavaScript app (React/Angular/Vue3). Build dynamic JSON forms without coding. Fully customizable, works with any backend, perfect for data-heavy apps. Learn more.