Synchronous `derived` stores should not trigger `require-stores-init`

[mcous]

Overview

I've been using the 3.0.0-next version in our Svelte project, and I'm really liking the new recommended set of rules! However, I've had to disable require-stores-init, because it triggered on nearly every single usage of derived.

Proposal

I think require-stores-init makes a lot of sense for writable and readable stores - it's goal seems to be to prevent an implicitly undefined store values. However, I think synchronous derived stores should not trigger the require-stores-init lint. If a derived store is synchronous, there is no risk that its value can be implicitly undefined, because it will run its callback synchronously when it gets its first subscriber.

If you agree with this change, I'm happy to contribute a PR!

Alternatives

Alternatively, I could imagine a configuration option that allows you to selectively enable or disable require-stores-init for each store type. However, I think enforcing this rule on asynchronous derived stores is valuable, so I prefer my proposal above.

[baseballyama]

It might be good to add an option like ignoreDerived (default is false).

[mcous]

Alternatively, with stores on the way out with Svelte 5, maybe it's simply better to remove require-stores-init from the recommended config. As currently defined, it's way too noisy in my real-life app to be useful.

ignoreDerived could be a pragmatic solution, but I don't think it's ideal for actual derived usage. In an ideal world, I do want to enforce an initial value on asynchronous derived stores:

const source = writable(0)

// synchronous, initial value not needed
const doubled = derived(source, ($source) => $source * 2)
 
// async., initial value needed
const doubledLater = derived(
  source,
  ($source, set) => {
    const id = setTimeout(() => set($source * 2), 1000)
    return () => clearTimeout(id)
  },
  0
)

[baseballyama]

I decided to remove it from the recommended config.👍