Spurious prop warnings with `+page.svelte` files

[stephenlrandall]

Describe the bug

On each route not using a load function, the console has

[Warning] <Page> was created with unknown prop 'data' (+page.svelte, line ...)
[Warning] <Page> was created with unknown prop 'errors' (+page.svelte, line ...)

These can be removed with the boilerplate

export let data;
export let errors;

in the +page.svelte files for those routes, but ideally the warnings would not be present unless the route has an associated load function.

Reproduction

Launch a SvelteKit app that includes routes which do not explicitly accept data or errors and check the console.

Logs

No response

System Info

System:
    OS: macOS 12.5
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 25.14 MB / 16.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.4.0 - ~/Library/pnpm/node
    npm: 8.12.1 - ~/Library/pnpm/npm
  Browsers:
    Safari: 15.6
  npmPackages:
    @sveltejs/adapter-static: 1.0.0-next.38 => 1.0.0-next.38 
    @sveltejs/kit: next => 1.0.0-next.411 
    svelte: ^3.49.0 => 3.49.0 
    vite: ^3.0.8 => 3.0.8

Severity

annoyance

Additional Information

No response

[dummdidumm]

This comes from a warning in Svelte core, which we IMHO could just remove. I also believe there is some (closed?) issue in this repo already noticing the same thing in a different context.

[sserdyuk]

The warning is helpful if you accidentally use the wrong binding on a component.

[benmccann]

sveltejs/svelte#4652 looks to be the issue in the Svelte repo. If we're going to be removing annoying warnings then I'd also nominate sveltejs/svelte#6325

[stephenlrandall]

I agree that the warning is useful in many cases (I've definitely caught a lot of issues thanks to those warnings), but only when I'm intentionally passing props around. If I don't have a +page.js in a route, I wouldn't expect data or errors to be passed in to the +page.svelte file and I wouldn't want to be warned that I'm missing some boilerplate export let data line for an unused data.

[Rich-Harris]

Agree, the warning should only appear if we're passing data or errors and they're not being received

[charbelnicolas]

I just upgraded to the new directory based routing and immediately noticed these annoying warnings!

+1 for removing the warnings

[dummdidumm]

The problem is - how do we leave the warning in place while silencing it for this use case? There would have to be some kind of runtime code involved. Two possibilities I can think of:

• Svelte exposes some kind of method that says "surpress the warnings of type X" and another one that says "stop surpressing warnings of type X", which we call before and after setting the props
• Svelte exposes some kind of info on its components in dev mode: "these are the props I expect", and SvelteKit's component uses that info to not pass data/errors in this case

[Zamiell]

Hello,

I am trying out SvelteKit for the first time. Upon running the provided "hello world" application with the counter, I am greeted with the spurious warnings on the console as described in the OP, which lead me to wading through this thread & sveltejs/svelte#4652 to try and get a handle on the situation.

This is not a great DX experience for new users. In the first minutes of using SvelteKit, I want to be building my app, not spending 15 minutes troubleshooting warnings. I'm posting here not to complain about the product, but rather to emphasize the importance of fixing this issue if SvelteKit is to retain new users.

[aradalvand]

Agree, the warning should only appear if we're passing data or errors and they're not being received

@Rich-Harris Perhaps not even in those cases, sometimes you're just returning data for the layout(s) to consume, you don't necessarily need it in the +page.svelte itself. And a warning in these cases would also be annoying.