feat: Fail prerendering for pages with mutative endpoints - fixes #3410 and #4252

[elliott-with-the-longest-name-on-github]

There are a few issues up around the confusing things that happen if a page is prerendered with mutative methods. Thankfully, this already errors at buildtime. However, it can be a bit of a footgun during development, as there's no warning what you're doing isn't valid until you try to build.

Added tests to make sure that, if a page should be prerendered (both a build-time and at runtime), it fails to do so with a descriptive message.

Fixes #3410 and fixes #4252.

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[elliott-with-the-longest-name-on-github]

Only downside of this that I can think of is that it'll only fail in dev when a server-side navigation occurs... but I don't think it's even possible to check for prerendering in the client. (A quick search of /packages/kit/src/runtime/client for "prerender" yields... 0 results.)

[Rich-Harris]

Thank you! There was one small change I needed to make — whereas in the page respond.js module we're checking whether we're actually prerendering at the moment, in the context of this PR we need to check whether we will prerender the page. In the first case we need to check state, but in the second (in e.g. dev mode) we need to check configuration.

Though thinking about it, state.prerender.default is now redundant... one sec...

[Rich-Harris]

I renamed state.prerender to state.prerendering to make the distinction clearer

[elliott-with-the-longest-name-on-github]

I renamed state.prerender to state.prerendering to make the distinction clearer

Thanks for renaming this. I knew there was something not making sense -- I thought maybe the prerender configuration was assigned to state. The new name is much clearer. 🙂

[Rich-Harris]

Ah whoops, looks like I broke something. Will look into it in a bit

[elliott-with-the-longest-name-on-github]

Bah, I looked into it for a bit, but I'm just not sure what's causing the issue. To save you some time, what's failing is a test in adapter-static that's making sure that when config.kit.prerender.default = false, pages that do not set export const prerender = true are not prerendered. For some reason, your last commit has made the prerendering process prerender index.html even though index.svelte does not export the prerendering attribute. I might have more time to look into it later.

[sarajw]

In case this is useful to you, my experiences:

I tried to update my blog that uses adapter-static the day before yesterday and it didn't build right - only one of my *.svelte pages (incidentally the simplest and most recently edited) was built into a *.html file.

I could open the site via that one rendered html page and - I'm not smart enough yet to understand how it all works - I guess the rest was populating itself via JS and it sort of worked. Apart from, the blog index page which complained about the JSON it was referencing having a rogue < in it at 0.

Anyway eventually I paid attention to some warnings I was getting in the terminal, saying I had to add
prerender: { default: true }
to svelte.config.js, and
"extends": "./.svelte-kit/tsconfig.json"
to jsconfig.json.

Then after that it built fine again. Are these lines I will have to change back once any fixes happen?

Thanks!

[elliott-with-the-longest-name-on-github]

Hey @sarajw! This is an unrelated issue and is actually intended behavior (unless there's something I'm not understanding about your issue). The terminal warnings you were seeing were trying to tell you this. For adapter-static, you must specify either kit.prerender.default = true (to prerender your site into static HTML) or adapter.fallback = <some_page> (to run in SPA mode). You can read more about both of these options in the adapter documentation -- and feel free to ask more questions in the SvelteKit help channel on Discord!

[sarajw]

Hey @sarajw! This is an unrelated issue and is actually intended behavior (unless there's something I'm not understanding about your issue). The terminal warnings you were seeing were trying to tell you this. For adapter-static, you must specify either kit.prerender.default = true (to prerender your site into static HTML) or adapter.fallback = <some_page> (to run in SPA mode). You can read more about both of these options in the adapter documentation -- and feel free to ask more questions in the SvelteKit help channel on Discord!

Absolutely fine if that's the case, it was just new and surprising. I had already posted in the Discord SvelteKit channel and not got much help or information.

[elliott-with-the-longest-name-on-github]

Absolutely fine if that's the case, it was just new and surprising. I had already posted in the Discord SvelteKit channel and not got much help or information.

And to be clear, I'm not saying you're not experiencing a bug -- just that I don't see any buggy behavior described in what you said. If you still think you are, please do create a repro and file an issue. 🙂 I personally think it's kind of weird that adapter-static doesn't "just work" without changing the mentioned settings, so it may be worth creating an issue to discuss ergonomics? 🤷

[Rich-Harris]

thank you!