Improve handling of compilerOptions.hydratable

[dominikg]

Describe the problem

Currently vite-plugin-svelte defaults to hydratable: true which is not the default for svelte and mostly due to the reason that it reliably works with and without ssr.

In situations where you don't use ssr and/or don't need clientside hydration, this leads to increased output size.

Describe the proposed solution

Only compile with hydratable: true when one of the following is true

• compilerOptions.hydratable is set to true in svelte config
• compilerOptions.hydratable is not defined in svelte config and viteConfig.ssr exists

Alternatives considered

stick with the current default. Users that want hydratable: false should set it in svelte config.

Importance

nice to have

[benmccann]

Sounds fine to me as long as we specify a default of true in SvelteKit still

[dominikg]

Explicit compilerOptions.hydratable in kit is a good idea if #121 doesn't work out.

It would work even with the current version of kit as it sets viteConfig.ssr.

That check isn't ideal (false positive if the config value is set but ssr isn't used), but the more correct/strict check

const isSSR = !!(viteEnv.command === 'serve' ? viteConfig.server.middlewareMode : viteConfig.build.ssr);

would be false for the build of client bundle, where hydratable is needed.

[bluwy]

I never realized hydratable was set to true by default all these time. Is it true for production builds too? I think this is a welcomed change.

[dominikg]

It would work even with the current version of kit as it sets viteConfig.ssr.

turns out i was wrong. the kit client build does not set it, and it breaks as expected. So this change would require kit to explicitly set it.

[dominikg]

PR in kit to set hydratable: sveltejs/kit#2024

using viteConfig.ssr turned out to be not the best idea as this is not always set when hydratable is needed. So instead of adding an automatic default that only works half of the time, vite-plugin-svelte will default to hydratable: false always

[dominikg]

released https://github.com/sveltejs/vite-plugin-svelte/blob/main/packages/vite-plugin-svelte/CHANGELOG.md#100-next15