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Closest I can think of is ttypescript
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SaaSHub
SaaSHub - Software Alternatives and Reviews. SaaSHub helps you find the best software and product alternatives
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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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