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Pattern matching doesn't require any kind of runtime support, and can be implemented through desugaring.

How so?

I believe there was/is a stage 0 proposal for pattern matching in ECMAScript

Stage 1 and yes, this one simply uses destructuring.

But it could use JavaScripts own types.

But it couldn't use TypeScripts types.

Why would it need runtime support? It would just desugar into an if-else chain.