fix: allow writable/readable empty initialization

[ignatiusmb]

Fixes #6291
Fixes #6345

Both writable and readable initialized without any arguments are already valid, but TS complains about it. This makes both allowed to be emptily initialized. A special exception to readable, although it's valid to use empty initialization (not sure why anyone would do this), when the first argument is passed, the callback is required to be defined as well (according to the docs).

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.

[dodiameer]

Looks like there aren't any problems with the changes!

[ignatiusmb]

This should now fix #6345 as well

[dummdidumm]

I think you need to update the docs some more, right now they tell you that the second argument is required, which would be no longer the case. Other than this this looks good 👍

[ignatiusmb]

Oops, nice catch. Done!

[dummdidumm]

Just to be extra safe: Could you add

• one test for a writable without an arg
• one test for a readable without an arg
• one test for a readable without an updater

to https://github.com/sveltejs/svelte/blob/master/test/store/index.js ? That would be great.

[ignatiusmb]

Good idea, done!

---

I was confused why npm run test -- -g store didn't produce the output, I took a look at GitHub pipelines as well and there's no sign of store. Turns out the test only globs .ts indexes, so I renamed it (seems to be the only one left behind as well).

svelte/test/test.ts

Line 11 in 3301b09

const test_folders = glob('*/index.ts', { cwd: 'test' });

[dummdidumm]

Thank you! 👍