onServerPrefetch with useId cause hydration issue if treeshaken client-side

[huang-julien]

Vue version

3.5.13

Link to minimal reproduction

https://stackblitz.com/edit/vue-ssr-example-bsqzmafk?file=app.js,client.js,server.js

Steps to reproduce

open your console and see there's a hydration mismatch.

Comp.vue's useId is not retturning the same value server side and client side

What is expected?

no hydration issue due to useId

What is actually happening?

The cause of this issue is that we run onServerPrefetch only server-side.

This happen in Nuxt because we treeshake onServerPrefetch in client-bundle and we also wrap onServerPrefetch with import.meta.server flag in useAsyncData composable.

In runtime, this results into the component being marked as async boudary only server side with markAsyncBoundary but not client side --> resulting to a hydration issue because useId returns different strings.

System Info

No response

Any additional comments?

Nuxt issue nuxt/nuxt#30289

[edison1105]

When rendering on the CSR, it checks if the component contains onServerPrefetch hook at

core/packages/runtime-core/src/component.ts

Lines 876 to 879 in 1755ac0

	if ((isAsyncSetup || instance.sp) && !isAsyncWrapper(instance)) {
	// async setup / serverPrefetch, mark as async boundary for useId()
	markAsyncBoundary(instance)
	}

If it does, it calls markAsynvBoundary to make it consistent with SSR behavior.

[huang-julien]

In Nuxt side, we could either treeshake only the callback of onServerPrefetch in client side bundle or add instance.sp = instance.sp || [].

But i'm not really in favour of the second one tho.

Or maybe can we leave it to vue: we remove the treeshaking of onServerPrefetch and maybe can the vue compiler do the treeshaking for client-side bundle ? WDYT ?

cc @danielroe

[cernymatej]

imho, it would be nice to tree-shake it on the Vue side

[danielroe]

I'm happy to remove treeshaking of onServerPrefetch, more generally, and leave it up to the vue compiler - but it's not quite that straightforward. In this case, the call to onServerPrefetch is in an entire separate if/then fork (because data fetching on the server and in the hydration process in the browser run differently).

  // Server side
  if (import.meta.server && fetchOnServer && options.immediate) {
    const promise = initialFetch()
    if (getCurrentInstance()) {
      onServerPrefetch(() => promise)
    } else {
      nuxtApp.hook('app:created', async () => { await promise })
    }
  }

https://github.com/nuxt/nuxt/blob/main/packages/nuxt/src/app/composables/asyncData.ts#L338-L347

In other words, we'd have to artificially add an 'onServerPrefetch' which awaits an empty promise in order to 'match'. More than happy to do that, but is there another way?

[huang-julien]

Yes, useAsyncData is a too specific case to be handled by vue-compiler. In the meantime, we can try the second option using an empty instance.sp = instance.sp || []
Or fully remove the treeshaking of onServerPrefetch and we try treeshaking the callback in Nuxt side ?

[edison1105]

Vue compiler cannot find all onServerPrefetch calls during compilation. Apart from the scenario mentioned by @danielroe, users may also call an externally imported function, which internally calls onServerPrefetch.