Add native-preview API for capturing .d.ts emit output

[hegrec]

Hey folks, I’ve been experimenting with the native-preview JS API for TS-go API mode, and I’m getting a lot of value from the virtual filesystem callbacks, especially overriding readFile for typechecking.

I’m curious whether there are plans to expose declaration emit through this API as well.

Today, as far as I can tell, the JS API can ask for declaration diagnostics, and the CLI can emit .d.ts files, but there does not seem to be an API-mode equivalent that lets a JS caller capture emitted .d.ts contents while still using the virtual filesystem callbacks.

The use case is an orchestration layer that already supplies source contents through readFile callbacks. I want to extend this system to capture generated declaration text for storage/caching without writing to the real filesystem.

Concretely, I’m wondering whether an API roughly like this would be in scope:

const emittedFiles = new Map<string, string>();

const api = new API({
  fs: {
    readFile: (filePath) => {
      // Optionally provide source contents from a virtual FS.
    },
    writeFile: (filePath, content) => {
      emittedFiles.set(filePath, content);
    },
  },
});

await api.emit({
  project: "/path/to/tsconfig.json",
  fileName: "foo.ts", // optional: omit for whole project
  emitOnly: "dts",
  compilerOptions: {
    declaration: true,
    emitDeclarationOnly: true,
    noCheck: true,
    declarationMap: false,
  },
});

[jakebailey]

Related is microsoft/typescript-go#3616

[hegrec]

Thanks Jake Bailey (@jakebailey)! Are there any timelines/plans to support such an API either through that PR or other means? Hoping to get a sense of what to expect down the line there regarding an API for emissions in general but especially in combination with the fs callbacks system. Happy to be an early tester of anything there and give feedback as well if there's a world where that is helpful.

[jakebailey]

Of course yes, but don't expect it for 7.0