feat: add Svelte v5-next support

[mcous]

Overview

This PR is an alternative approach to #284 (CC @yanick) that adds experimental support for svelte@next without breaking support for Svelte v3 nor v4. I think adding support for the prerelease version of Svelte v5 as soon as possible will be a really helpful move for everyone trying to migrate - myself very much included.

This PR requires the cleanup work I did in #314. Please see mcous#1 in my fork for the incremental diff from #314 to here. This PR is ready for review, but #314 should merge first

Issues encountered

• onMount and onDestroy are attached and called asynchronously
  • If you synchronously create and then destroy a component, neither onMount nor onDestroy will be called
  • Tests required a sprinkling of act()s to get passing again
• anchor option has been removed
  • Doesn't directly affect this library, we just have a test that exercises it
• Element.animate is required to run transitions
  • Doesn't directly affect this library
  • Does affect any users running transitions in tests, including our own test suite
  • Not sure what the right move is here; probably a documentation update
• happy-dom is pretty broken, probably a Svelte issue
  • See Happy-DOM on svelte5 branch causes PrettyFormatPluginError when rendering 1 line #319
  • See Svelte 5: happy-dom test fails with >2 components in a file sveltejs/svelte#10358

Work after this PR

• If we want to guarantee that onMount and onDestroy are called, we'll need to make render async so it can call tick() after render, which will be a breaking change
• Accordingly, I think we'll still want to issue a major bump that drops Svelte 3 and Node 16, maybe when Svelte 5 proper is released
• We can and should keep Svelte 4 support, in my opinion

[RobinKnipe]

Is there any consumer emulation as part of the CI flow?
I've setup a svelte5 project (with npx create svelte@latest) and chosen vitest support, then changed to this PR's commit:

 + @testing-library/svelte@git+ssh://github.com/mcous/svelte-testing-library#150335aac28f071710ca5771d4dc2a4f7363633c

 1 package installed [1.56s]

...but running a simple component test still fails to render 😞 I had hoped for more success seeing all the jsdom checks passing here, but I'm still seeing the same error as the latest release:

Instantiating a component with `new` is no longer valid in Svelte 5.

[yanick]

It'll soon be in a next release. I've just cut the very first testing of next this morning with some other changes, and I'll try to add in the v5 support in that branch later on. Hold tight, I'll try to report on it before the end of the day.

[mcous]

@RobinKnipe that smells like a dependency install issue, you might want to check if you have duplicate versions installed with npm why @testing-library/svelte (if I'm remembering that command properly).

Like @yanick said, you'll probably have a happier time if you wait for the prerelease version in npm. I've never messed around with installing this library via git so I don't really know what pitfalls and limitations exist, especially coming if it's coming from a fork rather than upstream

If you're curious what our CI does, check out release.yml in the workflows folder; it's fairly straightforward.

[RobinKnipe]

If it helps; some partial success in this test project. Noddy unit tests created for the 2 components from the default "create svelte" setup (one working, one failing).

This is using the changes in this PR:

svelte5$ npm why @testing-library/svelte
@testing-library/svelte@0.0.0-semantically-released dev
node_modules/@testing-library/svelte
  dev @testing-library/svelte@"git+ssh://github.com/mcous/svelte-testing-library#150335aac28f071710ca5771d4dc2a4f7363633c" from the root project

[mcous]

@RobinKnipe your example repo has three problems, all interacting/hiding each other, that are causing that <Header> failure. All three issues are about how to test SvelteKit apps, not Svelte v5. The exact same failure happens if you switch your repo to Svelte 4.

• Vite is configured with the vanilla Svelte plugin rather than SvelteKit, which hides...
• Vite is not configured to resolve the browser bundle of Svelte, which hides...
• The SvelteKit $page store will be empty in unit tests, so your test crashes on:

  TypeError: Cannot read properties of undefined (reading 'pathname')
   ❯ Object.hydrate [as h] src/routes/Header.svelte:19:32
       17|   </svg>
       18|   <ul>
       19|    <li aria-current={$page.url.pathname === '/' ? 'page' : undefined}>
         |                                ^
       20|     <a href="/">Home</a>
       21|    </li>
  

Since these are not issues with this library nor Svelte v5, if you have any additional questions, please open a separate discussion if you need help testing a SvelteKit app

[yanick]

Basing on 'next', and I'm going to push the conflict resolutions in 2 minutes.

[yanick]

@mcous You good with me merging this bad boy into next and seeing if it triggers our first next-type release to npm?

[mcous]

@yanick oh boy 😬, no time like the present

The only thing giving me pause is that our next branch is not configured as a prerelease in release.yml. So I think merging may bump to 5.0.0 proper and release it on the next tag.

Not sure what your desires are on 5.0.0 vs 5.0.0-next.0 (or whatever). Either way I'm happy with what you choose! I'm fairly confident with this PR given the test matrix and the low risk to harming Svelte v4 support

[yanick]

The only thing giving me pause is that our next branch is not configured as a prerelease in release.yml. So I think merging may bump to 5.0.0 proper and release it on the next tag.

Oh yeah, you're right. next and main are regular releases, and alpha and beta branches are prereleases.

Not sure what your desires are on 5.0.0 vs 5.0.0-next.0 (or whatever). Either way I'm happy with what you choose! I'm fairly confident with this PR given the test matrix and the low risk to harming Svelte v4 support

Hmmm, it's true that with the test suite, the chances that we'll utterly bugger up everything are slim (and yes, I just jinxed us by saying that). But today is a beautiful day to di-- I mean, deploy, so let's Leeroy Jenkins this puppy out and see what happens. >.>

[github-actions]

🎉 This PR is included in version 4.2.0 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[mcous]

Hey, cool! Looks like it missed the breaking fix! for the rerender changes, but I'd like to land a few more pedantically breaking changes into next anyway (see #312 and #313) so we'll see what happens

[RobinKnipe]

@mcous ah it's SvelteKit related!? Ok, I'll try and refine my testing to Svelte only examples. Thanks!