[Experiment] feat(13626): Type annotations for default export

[a-tarasyuk]

Fixes #13626

The compiler supports checking JsDoc type annotations for default exports (Playground, #47946), however, in TypeScript files, users are not allowed to add a type annotation to have this kind of validation.

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #13626. If you can get it accepted, this PR will have a better chance of being reviewed.

[sandersn]

From reading the issue, it sounds like there are open questions, at least for how to parse the type. Can you explain what the problem is and how this PR solves it? (If there are too many questions it might be better to hold off on this PR and settle them on the original issue.)

[a-tarasyuk]

Can you explain what the problem is

Using JsDoc, we can use the type annotation for the default export like this

// @filename: /a.js
/** @type {string} */
export default 1;
               ^ Type 'number' is not assignable to type 'string'.

However, in TypeScript, we cannot directly add type annotation for the default export, only through alternative constructs like additional variables. Based on reactions and feedback, this limitation adds confusion (wat?) for users.

export default: number ...
              ^ Expression expected

and how this PR solves it?

This is a PoC that allows using type annotation for the export default in the following way

default export: <type> = <expression>

Why did I use =?
As far as I know, TypeScript ignores <space> in type annotations. The following examples are completely valid

const a: number   []   [] = [];
const b:   {}   [] = [];

I think it would be weird to disallow using <space> only in type annotations for export default.

export default: number   []   []   [];
                ^^^^^^^^^^^^^^^^

If there are too many questions

I think we need to find the best way to separate type annotation and expression.

1. Can we use : to add type annotation?
   I think, yes. At least to follow the same user experience.
2. Can we use = to separate type annotation and expression?
   Not sure.
3. What about <>? However, this seems more of an assertion.

export default<number> 1; 
export default<number[]> []; 

@sandersn Do I need to add this answer to the issue and close PR? Maybe @RyanCavanaugh has thoughts about that case.

[sandersn]

Do I need to add this answer to the #13626 and close PR?

I think so. The code changes are pretty small and the hard part is getting people to agree on the design, which is better done on an issue. Though, I don't know, maybe starting a new issue with your complete proposal would be easier to follow than appending it to the original.