Component unmount results in undefined variables in closures

[staab]

Describe the bug

If a function which closes over state received via props is called after that component unmounts, the closed over value becomes undefined. This is a breaking change compared to svelte 4, but I don't see it called out in the migration guide or anywhere else in the documentation. Fixing this behavior to allow passing of closures around requires awkward/annoying/error prone value re-assignment.

Reproduction

Here is a Stack Overflow question that goes into more detail: https://stackoverflow.com/questions/79446266/svelte-5-props-are-undefined-after-unmount

Reproduction source code: https://github.com/staab/svelte5-closure-bug
Reproduction demo: https://svelte5-closure-bug.onrender.com/

Logs

init 1
update undefined
value is undefined: undefined

System Info

System:
    OS: macOS 15.3
    CPU: (8) arm64 Apple M1
    Memory: 74.69 MB / 8.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.20.6 - ~/.nvm/versions/node/v18.20.6/bin/node
    npm: 10.8.2 - ~/.nvm/versions/node/v18.20.6/bin/npm
    pnpm: 9.15.0 - ~/Library/pnpm/pnpm
    bun: 1.2.2 - ~/.nvm/versions/node/v18.20.6/bin/bun
  Browsers:
    Brave Browser: 132.1.74.50
    Chrome: 133.0.6943.98
    Safari: 18.3
  npmPackages:
    svelte: ^5.0.0 => 5.20.1

Severity

annoyance (additional context: I was a big fan of svelte 4. For this reason and others, I don't plan to use svelte for any new projects)

[staab]

So far this is the cleanest solution I've found to my problem:

const closure = ((value) => {
  console.log(value)
}).bind(undefined, value)

But I don't love this, for obvious reasons.

[Prinzhorn]

#14707 also #14725 #14911 #14950

[dummdidumm]

Closing as duplicate of #14725 (the underlying reason is the same)