[Astro + Svelte + Cloudflare] Cannot build Svelte component that contains $onMount() with Cloudflare adapter

[GoodLucky777]

Astro Info

Astro                    v5.0.5
Node                     v18.20.3
System                   Linux (x64)
Package Manager          unknown
Output                   static
Adapter                  @astrojs/cloudflare
Integrations             @astrojs/svelte

Describe the Bug

If I try to build the Astro website that contains Svelte component with $onMount() and Cloudflare adapter, the build error will occurs like this.

Build error:

$ npm run build

prerendering static routes 
01:36:03 ▶ src/pages/index.astro
01:36:03   └─ /index.htmlhttps://svelte.dev/e/lifecycle_outside_component
  Stack trace:
    at lifecycle_outside_component (file:///home/projects/github-memgm1vc/dist/_worker.js/pages/index.astro.mjs?time=1734280563043:32:9)
    at Test (file:///home/projects/github-memgm1vc/dist/_worker.js/pages/index.astro.mjs?time=1734280563043:69:9)
    at renderToStaticMarkup (file:///home/projects/github-memgm1vc/dist/_worker.js/chunks/_@astro-renderers_BFYr5CxZ.mjs:1251:23)

It doesn't occurs when I use NodeJS adapter. It seems it is occurring after @astrojs/cloudflare@12.1.0 update.

What's the expected result?

No build errors.

Link to Minimal Reproducible Example

https://stackblitz.com/edit/astro-svelte-cloudflare-build-error

Participation

• I am willing to submit a pull request for this issue.

[rognstadragnar]

Seeing the same thing. It also fails when using setContext with the same error message.

Previously this error was caused because the Vite resolve conditions were including node. A related issue when using Vue was attempted fixed in #476 and released in the latest @astrojs/cloudflare@12.1.0.

Unfortunately this did not fix the Svelte issue, as it introduced the browser resolve condition for the server environment. This now leads Svelte to pick its browser version when running on the server.

[bluwy]

Can you try Svelte 5.15.0 in your project if it fixes the issue? I've sent a change to add the worker condition, so it should work better now. If it doesn't cause other build tooling regressions that requires a revert there, I think it would be the best fix given the case here.

[GoodLucky777]

Can you try Svelte 5.15.0 in your project if it fixes the issue? I've sent a change to add the worker condition, so it should work better now. If it doesn't cause other build tooling regressions that requires a revert there, I think it would be the best fix given the case here.

It seems the build error is fixed in Svelte 5.15.0.

[bluwy]

I'll close this for now as fixed then. Thanks for testing!