Svelte 5 HMR removes extra properties added to component instance after mount()

[Mtn-View]

Describe the bug

Abstract State Router / Svelte State Renderer render the Svelte component, and then add extra properties on the component instance to later use in other parts of the lifecycle.

In Svelte 4, triggering HMR on one of these components worked fine, and the extra properties stayed intact. In Svelte 5, they get removed, and this breaks the router after HMR. Reloading the page fixes this issue, but I'd prefer to keep using HMR in Svelte 5.

I also posted about this in the Svelte discord, if you want more context: https://discord.com/channels/457912077277855764/1323699302073372843

Reproduction

https://github.com/Mtn-View/svelte-5-hmr-issue

• git clone
• npm i
• npx vite
• Change App.svelte and save to trigger HMR
• Click one of the links on the page, and you will get the error.

Logs

No response

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (12) x64 Intel(R) Core(TM) 7 150U
    Memory: 3.06 GB / 15.70 GB
  Binaries:
    Node: 20.18.1 - C:\Program Files\nodejs\node.EXE
    npm: 10.8.2 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.15.2 - C:\Program Files\nodejs\pnpm.CMD
  Browsers:
    Edge: Chromium (131.0.2903.63)
    Internet Explorer: 11.0.22621.3527
  npmPackages:
    svelte: ^5.15.0 => 5.16.1

Severity

annoyance

[trueadm]

This is an incompatibility with svelte-state-renderer and abstract-state-router in the fact they are using internals around how HMR worked in Svelte 3/4. This is no longer supported in Svelte 5, so we recommend speaking to the library author to upgrade support for Svelte 5.