The TypeScript compiler can generate type declarations from JSDoc comments in JavaScript sources. I’ve been using it to provide TypeScript support for Node libraries that are still maintained as pure JavaScript.
While TypeScript has quickly taken over front-end library development, many Node libraries are still maintained as pure JS. That’s partly due to differences in lifecycles and codebase maturity, but there’s also a technical reason for the difference: in the front-end TypeScript is largely replacing an existing transpile step, so it’s a natural evolution of longstanding development toolchains. Since Node codebases don’t contend with browser support, they have no natural need for such a transpile step. That makes transpilation – and its associated inconveniences to testing and debugging – a new friction when adopting TypeScript in Node. (This is one of Deno’s draws: TypeScript support by default means less tooling to argue with.)
Regardless, type declaration files are a common feature request for Node libraries. The compiler’s JSDoc support seems a good solution: it allows library contributors to work in pure JS, toolchains untampered, with only a publish script in package.json to generate .d.ts files for distribution:
"publish": "tsc --allowJs --declaration --emitDeclarationOnly --outDir .",
That satisfies consumers’ expectations for type support without disrupting development, adding only a short automatic step to distribution and a single shallow dependency for typescript itself.