Allow rest parameters as end of endpoint route

[romaricpascal]

Is your feature request related to a problem? Please describe.
I'm trying to organise each route in my sveltekit app with each .svelte route fetching their data from a corresponding .json.js exportint a get() function.

It's all working nicely, until I've had to create a route with a terminal rest parameter:

routes
-- concepts
---- [...conceptPath].svelte

Sveltekit was pretty happy with the rest path for the .svelte route. However, it rejected a [...conceptPath].json.js with:

rest parameter must be a standalone segment

It seems happy with the rest parameter set as a folder, though:

routes
-- concepts
---- [...conceptPath]
------- index.json.js
------- index.svelte

While this works, it'll lead to a good deal of index.svelte/index.json files

Wrote a bit soon regarding the above structure working, the server seems to favour the index.json.js route over the index.svelte, and I'm getting JSON instead of HTML.

This was circumvented again with setting the .json.js to a different path than index.

-- concepts
---- [...conceptPath]
------ load.json.js
------ index.svelte

It makes the delegation from the .svelte file to .json.js a little less generic.

Describe the solution you'd like
It'd be great to be able to declare endpoints with rest parameters. If there's a technical limitation to it, though, having the situation highlighted in the documentation would work too.

Describe alternatives you've considered
Placing the rest parameter as a folder provides a workaround.

How important is this feature to you?
It's more a mild annoyance than anything as there's a workaround. Having some clarification in the documentation could help people wonder why their route is rejected though.

[andrewspode]

This issue is making it impossible for us to migration from Sapper as the routing has changed. The rules for a .svelte file should be the same as the rules for a .json.js file.

[stephane-vanraes]

The problem is the rules are the same namely that a rest parameter should be the only thing appearing in the name part of a route (name.extension) your name here would be [...concept].json which is invalid. Note that a route like a-[...slug].svelte would also be invalid.

One workaround for this is to make use of the fallthrough mechanisms of SvelteKit with the following two routes:

• [...slug].js
• [...slug].svelte

(without the .json part!)

Now, because the specifity rules say the .svelte should be executed first you have to make sure that if the route ends with .json you go through the next match, this is fairly easy:

export async function load({ page }) {
  if (page.params.endsWith('.json')) return;
  // rest of your code goes here
}

This will basically short-circuit the route and have the .js one kick in.

[andrewspode]

Aha, you're correct. The reason I was having a difference is because of the .json portion of the file structure. I was delving into the code myself and couldn't understand why it was matching differently - it's not.

This is still a deprecation in functionality on the routing in Sapper though - and there's not been an explanation as to why?

There are definitely scenarios where the above issues can't be worked around with your suggestion, and more importantly - not a clear migration path without a change in URL structure.