[dart2js] Produce TypeScript type annotations for compiled files

[TheOneWithTheBraid]

Dart as of now supports web by using AOT and JIT compiling. The AOT compiler already produces proper source maps.

I would like to propose to extend this to TypeScript-compatible type annotations too.

By using the allowInterloop function from dart:js, it is already possible to interloop a compiled dart library by native JS. Unfortunately, this is not easily implementable with TypeScript because all members are either any (dynamic) or the developers are required to manually create a .d.ts file for type annotations.

I would propose to either add a compiler option to produce corresponding .d.ts files (likely) or to add a two-stage compiler option compiling to TypeScript instead of ECMA script (unlikely).

As both Dart and TypeScript have a similar typing system, the effort should be comparably low.

[sigmundch]

Thanks for reaching out and the suggestion!

Note: given that the goal in this bug is similar to #46378 and #37617, I'm marking this as a duplicate.

As you mentioned in your comment - producing valid TypeScript for all Dart code is a complex task. We had discussions about it in the past, especially in the context of the DDC modular compiler, which is less sophisticated than the current optimizing deployment compiler (dart2js). But even there we felt it was a big undertaking.

Unfortunately, even producing .d.ts files is also in tension with other requirements we have. While we support interoperability between the Dart and JS - our design space is to be very intentional of where the interoperability takes place and properly export APIs that are exposed. That allows a smoother evolution of our compilers and enables us to adapt our output representation over time. If every API we generate has a .d.ts file, it will be very hard for us to change calling conventions or even type representations (which for example we are currently doing in some parts of our system).

I should point out that there is one thing that can be done today. It should be possible to generate some of the boilerplate of the JS-interop facade. The js_facade_gen tool, for example, takes .d.ts definitions and can be used to generate dart JSInterop APIs. It is also possible to write code generators that walk the JSInteorp Dart declarations, and generate .d.ts JS files to export the JavaScript API being used.

I sense from your comment that you have a higher need to consume Dart from TS, rather than the other way around. So far, JSInterop makes it easier to consume JavaScript in Dart, but it doesn't make it easy to consume Dart from JavaScript, so the process is more tedious unfortunately :(. This is an area we are about to start looking into as a team.