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by rtsao
710 days ago
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I've heard of `isolatedDeclarations` in TS 5.5, but this post left me with more questions than answers because it conflates so many distinct things (TS 5.5, Deno, JSR). I'll try my best to break it down succinctly: 1) Historically, .d.ts generation is slow because it requires the full TypeScript type checker in order to correctly handle all cases (e.g. exported functions with inferred return types) 2) However, if types were to be fully explicit, then .d.ts generation could be performed without a full type checker (i.e. by a faster compiler via mere syntax transformation) 3) The isolatedDeclarations flag causes the TS compiler to trigger errors when any exports would require inference in order to generate .d.ts 4) Thus, the isolatedDeclarations flag can be used to guarantee compatibility with faster, simpler .d.ts generation using syntax-based transformation 5) A consequence of simpler .d.ts generation is that it can be trivially done in parallel, which can be helpful for large monorepos |
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You're spot on with all of your points. The isolated declarations feature forces your code to be written in a way that creating the .d.ts files is a mere exercise of stripping syntax and can be done easily without invoking the tsc compiler.
For runtimes like Deno which support running TS natively (disclaimer: I work for Deno) you never had to care about creating .d.ts files when publishing your package to any of the Deno registries: previously /x, now JSR. In the background though we've always tried to feed the tsc compiler in Deno something like .d.ts files though as it's quicker for type checking purposes.