New rule: require-strict-events

[marekdedic]

Motivation

Hi,
when using TypeScript with svelte, component events aren't automatically typed - e. g. you can have an on:nonexistent={() => {}} handle for a non-existent event. However, svelte does check this, if the opening script tag of the child component has the strictEvents attribute. (<script lang="ts" strictEvents>).

I propose to add a rule that would enforce this.

Description

The rule should enforce that components have the strictEvents attribute present in their opening svelte tag.

I propose several an option to choose for which components the attribute should be enforced:

1. Require the attribute on all components. This makes sense since for a component that does not dispatch any event, the attribute still makes svelte check that no event listeners are present. I propose that this should be the default.
2. Only check components that actually dispatch an event - this would probably be more complicated to implement

Additional note: For some components that have event mixins, the strictEvents attribute does not work and you have to declare the $$Events interface. In that case, I think the rule shouldn't fire - so effectively, it would require either the strictEvents attribute or a manually declared interface $$Events for the component.

Examples

<!-- ✓ GOOD -->
<script lang="ts" strictEvents>
</script>

<script lang="ts">
  interface $$Events {}
</script>

<!-- ✗ BAD -->
<script lang="ts">
</script>

Additional comments

The relevant RFC.

[ota-meshi]

Thanks for the rule suggestions.
That rule sounds good to me, but it looks like the RFCs haven't been merged yet.
New rule should be marked as experimental until the RFC is merged.
sveltejs/rfcs#38

[marekdedic]

Hi,
I am honestly not sure how the RFC process works with svelte - the features mentioned in the RFC aren't mentioned in the docs for example, however, they do work for me with the current stable release.

If they're experimental, though, the rule should be too.

Thanks!

[marekdedic]

Relevant PR: sveltejs/language-tools#1054

[marekdedic]

Hi,
what exactly do you mean by "experimental" rule? I see no such option in eslint rule metadata...

Do you mean just a warning in the readme and a separate section in the rule list?

Thanks :)

[ota-meshi]

I think it would be nice to prefix the rule name with "experimental-". When it becomes GA, we will change the rule name and release it.