onDestroy doesnt update global stores value (at the same onDestroy function).

[Tichss]

Describe the bug

onDestroy doesnt update global stores value (at the same onDestroy function).

• it works with onMount.
• it works with local state rune.
• it was working with svelte4.

Reproduction

open link: https://svelte.dev/playground/6acd83b2792743c6addd512ed492cdfb?version=5.16.6
press button 2 times to mount and destroy component.
check the logs.

Logs

mounted, before val++ 0 expected: 0
mounted, after val++ 1 expected: 1
destroyed, before val-- 1 expected: 1
destroyed, after val-- 1 expected: 0 <-- this is the problem
destroyed, before localVal-- 0 expected: 0
destroyed, after localVal-- -1 expected: -1
destroyed, after val-- (setTimeout) 1 expected: 0

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (8) x64 Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
    Memory: 4.41 GB / 15.87 GB
  Binaries:
    Node: 20.11.1 - C:\Program Files\nodejs\node.EXE     
    npm: 8.19.2 - C:\Program Files\nodejs\npm.CMD        
  Browsers:
    Edge: Chromium (127.0.2651.74)
    Internet Explorer: 11.0.19041.4355

Severity

annoyance

[Prinzhorn]

This sounds similar to #14707 and #14911 in the sense that after a component unmounts the behavior of props and state becomes undefined or unpredictable.

[paoloricciuti]

It is kinda related but it's different...the problem is that when onDestroy runs the teardown of the stores has already happened. I wonder if we could simply delay the teardown to the next microtask to allow for onDestroy to run before. I'll have to investigate this and the consequences.

[Prinzhorn]

I wonder if we could simply delay the teardown to the next microtask to allow for onDestroy to run before

There's a setTimeout. The timeout callback would then still not get updates that happen in that timeframe.

[paoloricciuti]

I wonder if we could simply delay the teardown to the next microtask to allow for onDestroy to run before

There's a setTimeout. The timeout callback would then still not get updates that happen in that timeframe.

Oh true...this complicates things very much 😄

[Leonidaz]

So, this is definitely a departure from Svelte 4 behavior.

Svelte 4 repl

If you use get(val) to read the store's value vs $val in the svelte 5 repl, then it works as expected.

repl with get()