Svelte 5: undefined error stops the build

[mulder999]

I have a fairly large SvelteKit static SSR website that I attempted to migrate to Svelte 5 using npx sv migrate svelte-5. While the site works without migration, I opted for the automatic migration to potentially save time and effort.

However, when running the npm run build command, I encountered the following error, which doesn't provide enough information to help me resolve the issue:

> npm run build

> [email protected] build
> vite build

10:32:30 [vite-plugin-svelte] WARNING: The following packages have a svelte field in their package.json but no exports condition for svelte.

@splidejs/[email protected]
@splidejs/[email protected]

Please see https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#missing-exports-condition for details.        
vite v5.4.10 building SSR bundle for production...
✓ 30 modules transformed.
x Build failed in 393ms
error during build:
undefined

Is there a way to get more detailed output during the build process? Adding the --debug flag to vite build only dumps the vite.config file without offering any additional helpful insights.

[zuckschwerdt]

I just had the same experience (only upgrading Svelte, without migration). Turns out I had an onload="…" attribute somewhere that triggered this error during build: undefined. I swapped that for on:load|once={…} and it works fine now -- perhaps check for any on…="…" attributes in your component html?

[mulder999]

Thanks for hint.

However, using a divide and conquer approach I found the culprit was the use of space in the brackets of { @html text }.