Svelte 5 legacy interop bug: Uncaught TypeError: Cannot read properties of undefined (reading 'before') #15423
Description
Describe the bug
Seems like some niche combination of stores and component updates may lead to a runtime error.
We noticed it when we upgraded our site to Svelte 5. Upon page navigation, JS error'd out (see Logs below), blocking other critical runtime code.
This error does not seem to be present in dev mode, but only occurs in build && preview or on e.g. Vercel.
The requirements for this bug seem rather niche. We eventually found a workaround in our case (migrating more components to runes, and moving off of $app/stores). Unsure if a bug fix is worth it in this case(?).
Reproduction
https://stackblitz.com/edit/svelte-5-legacy-nav-update-bug?file=md!README.md
npm run build && npm run preview- (the bug is not present in
dev!) - (in the repro above, this will be run automatically)
- (the bug is not present in
- open the client console
- nagivate to another page
- (e.g. click any of the header links)
- inspect your console
- Expected: No console errors
- Actual:
Uncaught TypeError: Cannot read properties of undefined (reading 'before') at L (Doj7iJtn.js:1:839) at ks (0.Ck8LqK_p.js:2:3042) at 0.Ck8LqK_p.js:2:3658 at BBycYa5G.js:1:323 at Ut (C6bb17t1.js:1:8885) at ft (C6bb17t1.js:1:9715) at Sn (C6bb17t1.js:1:10628) at Ht (C6bb17t1.js:1:10041)
In some instances this may also block further page navigation (e.g. navigating to a third page).
In this repro, performing any of the following steps seems to bypass this issue:
- migrate
LdJsonScript.svelteto runes - migrate
meta.tsto runes and relovate into+layout.svelte - remove
<svelte:head>(i.e. render into regular<body>)
Logs
Uncaught TypeError: Cannot read properties of undefined (reading 'before')
at L (N6OWQcpE.js:1:841)
at Rs (0.DYl3NbUt.js:2:3042)
at 0.DYl3NbUt.js:2:3658
at CA0ixJbY.js:1:2699
at Ut (CS1pTN2Y.js:1:8858)
at ft (CS1pTN2Y.js:1:9688)
at Rn (CS1pTN2Y.js:1:10601)
at Ht (CS1pTN2Y.js:1:10014)System Info
System:
OS: Linux 5.0 undefined
CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
Memory: 0 Bytes / 0 Bytes
Shell: 1.0 - /bin/jsh
Binaries:
Node: 18.20.3 - /usr/local/bin/node
Yarn: 1.22.19 - /usr/local/bin/yarn
npm: 10.2.3 - /usr/local/bin/npm
pnpm: 8.15.6 - /usr/local/bin/pnpm
npmPackages:
svelte: ^5.0.0 => 5.20.5Severity
annoyance