Explicit fallthrough

[nhe23]

This PR makes fallthrough explicit. Only if {fallthrough: true} is returned by a load function or an endpoint the fallthrough mechanism is triggered. This way it is more clear when fallthrough is applied.
Fixes #2440 by allowing the same fallthrough logic that is used for page components.

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[Rich-Harris]

Thank you. My first reaction was that this could be confusing — it's much more likely that you'd simply forget to return something from load than that you'd want a layout to fall through.

But you could easily argue that the same is true for pages and endpoints. I've definitely been caught out by that once or twice (i.e. 404s that I wasn't expecting).

So I was wondering if we shouldn't make it explicit — make the return value required (i.e. load and endpoint handlers must return something), and make fallthroughs explicit:

<script context="module">
  async function load({ params }) {
    if (params.foo !== 'okay') {
      return { fallthrough: true };
    }
    return {};
  }
</script>

Thoughts?

[nhe23]

The implicit fallthrough when nothing is returned is indeed a bit confusing. I'll change that to your proposed solution. When fallthrough is returned all other properties won't be considered. Should this be reflected in the return type of load and endpoints? So for load either the normal load output is returned or {fallthrough:true}. The type could look like this:

export interface LoadOutput<
	Props extends Record<string, any> = Record<string, any>,
	Stuff extends Record<string, any> = Record<string, any>
> {
	status?: number;
	error?: string | Error;
	redirect?: string;
	props?: Props;
	stuff?: Stuff;
	maxage?: number;
} | {fallthrough: boolean }

Or is it enough to point this behavior out in the docs?

[dummdidumm]

It definitely should be reflected in the types 👍

[Rich-Harris]

It should be | { fallthrough: true } rather than | { fallthrough: boolean } I think (there's no reason you'd have false), but other than that yeah. Similarly for RequestHandler:

kit/packages/kit/types/endpoint.d.ts

Lines 12 to 18 in a59c89f

	export interface RequestHandler<
	Locals = Record<string, any>,
	Input = unknown,
	Output extends DefaultBody = DefaultBody
	> {
	(request: ServerRequest<Locals, Input>): MaybePromise<void | EndpointOutput<Output>>;
	}

[nhe23]

Fallthrough is now explicit as suggested by @Rich-Harris. In order to reflect this in the types I added an Either type to the helpers, because with Union Types only the values that the types have in common are directly accessible. So it would have been necessary to first check the returned type before accessing returned properties.

[Rich-Harris]

Thanks, this is looking good — just a couple of small things I found

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[Rich-Harris]

thank you!