Hitting a ReferenceError in `loadAndParseVirtualFilePageEntry` on versions 0.4.252 and 0.4.253

[JasonObeid]

Hello! Wondering if you might have any thoughts on what could be happening here?

Stacktrace

ReferenceError: Cannot access 'configValuesSerialized' before initialization.
../../../../node_modules/.pnpm/vike@0.4.253_react-streaming@0.4.15_react-dom@19.2.3_react@19.2.3__react@19.2.3__vite@8_68f134521940ab55d10c184b450f2feb/node_modules/vike/dist/shared-server-client/page-configs/loadAndParseVirtualFilePageEntry.js:19:26 (Pse)
    const configValues = parseVirtualFileExportsPageEntry(virtualFileExportsPageEntry);
../../../../node_modules/.pnpm/vike@0.4.253_react-streaming@0.4.15_react-dom@19.2.3_react@19.2.3__react@19.2.3__vite@8_68f134521940ab55d10c184b450f2feb/node_modules/vike/dist/shared-server-client/page-configs/loadAndParseVirtualFilePageEntry.js:25:54 (Fse)
    const configValues = parseConfigValuesSerialized(virtualFileExportsPageEntry.configValuesSerialized);

Description

• Some time after upgrading vike to 0.4.252 and 0.4.253, we started seeing this runtime error specifically on Safari / WebKit browsers.
• It appears to only occur on page load -> click anchor to do client-side navigation.
• When trying to reproduce, it happens inconsistently, maybe ~10% of the time I do the above ^.
• It doesn't appear to be an issue during local development.

Dependency versions

• vike: 0.4.253
• react-streaming: 0.4.15
• react: 19.2.3
• react-dom: 19.2.3
• vite: 8.0.0-beta.11 at onset (also observed later with beta.13 and beta.15)

Thanks for the help!

[brillout]

According to Claude:

---

Root cause: Safari WebKit TDZ bug with dynamic import()

The error Cannot access 'configValuesSerialized' before initialization is a Temporal Dead Zone (TDZ) ReferenceError — not a logic bug in user code, but a Safari/WebKit-specific quirk with ES module evaluation.

What happens

In loadAndParseVirtualFilePageEntry, vike dynamically imports the virtual page entry file:

const moduleExports = await import(virtualFileId)
// ...
moduleExports.configValuesSerialized  // ← throws ReferenceError in Safari

The virtual page entry file looks like this in the production build:

import { onRenderClient, ... } from '../chunks/chunk-B-qUESMw.js'  // already cached
import '../chunks/chunk-BWSb-mTJ.js'                                // already cached

const configValuesSerialized = { onRenderClient, ... }
export { configValuesSerialized }

In ES modules, export const creates a live binding on the module namespace object. Accessing it through the namespace (moduleExports.configValuesSerialized) throws a ReferenceError if the module body hasn't executed yet and the binding is still in TDZ.

Why Safari specifically

The ES spec says import() should resolve only after the module is fully evaluated. V8 (Chrome) and SpiderMonkey (Firefox) follow this correctly.

WebKit takes a shortcut: when all of a module's static imports are already in the module cache (e.g. chunk-B-qUESMw.js was loaded for the initial page), Safari can resolve the import() promise immediately after linking (wiring up the static deps) — before evaluating (actually running the module body where const configValuesSerialized = {...} is assigned). The live binding exists on the namespace object but is still in TDZ.

Why only on client-side navigation (not initial page load)

On initial page load, the shared chunks haven't been loaded yet, so the page entry module and all its deps are evaluated together in the normal sequence. On navigation, those shared chunks are already in the module cache — which is precisely the condition that triggers WebKit's early-resolve shortcut.

Why intermittent (~10%)

It depends on whether the shortcut is taken, which can vary with Safari version, system load, and whether a prefetch already started loading the module (vike's prefetchAssets calls loadAndParseVirtualFilePageEntry on hover, which can race with the actual navigation's import() call).

Fix

In loadAndParseVirtualFilePageEntry, wrap the access in a try/catch. On ReferenceError, yield to the event loop (setTimeout) to give the module body time to execute, then retry:

try {
  configValues = parseVirtualFileExportsPageEntry(virtualFileExportsPageEntry)
} catch (e) {
  if (!(e instanceof ReferenceError)) throw e
  // Yield so WebKit can finish evaluating the module body
  await new Promise<void>((resolve) => setTimeout(resolve))
  configValues = parseVirtualFileExportsPageEntry(virtualFileExportsPageEntry)
}

After the setTimeout, the module body has had a chance to run. The same moduleExports namespace object is reused — live bindings are a live view into the module, so once the module finishes evaluating, the binding becomes accessible through the same reference.

[brillout]

Claude:

---

Related

SvelteKit hit the exact same bug 16 days ago: sveltejs/kit#15287 — same error pattern, same conditions (Safari 26, client-side navigation, first visit only, doesn't happen if you hover long enough to preload). This appears to be a Safari 26 regression affecting multiple frameworks simultaneously. No WebKit bug tracker entry has been found yet.

[brillout]

Safari 26, released in September 2025 alongside iOS 26 and macOS Tahoe

[brillout]

I'll merge the fix (#3122) after GitHub Actions isn't down anymore.

[JasonObeid]

Awesome, thank you for the quick response and fix @brillout!

Once there's a release here I will give it a try and report back.

[brillout]

Fix pre-released as vike@0.4.253-commit-1d110cc.

By the way, we're collecting testimonials — contribution welcome to share 1-2 sentences about why you chose Vike.

[JasonObeid]

Following up here, I'm now seeing little to no errors come through for this.

I think the fix is working as intended!

I'm curious to see if this fully eliminates the error, or if it just made it very unlikely to occur. I can follow up again in a few days if I have any new observations.

[brillout]

👍 Keep me updated. We might indeed need to adjust the fix to definitely resolve the issue instead of merely reducing its occurrence. Let's see.