Svelte 5: $derived reacitvity breaks for mapped arrays that are reusing class instances

[ampatspell]

Describe the bug

Looks like if models (Source) are wrapped in $derived which contains some kind of caching strategy to reuse wrappers (Target), wrappers looses reactivity.

let cache = new WeakMap<Source, Target>();

let targets = $derived.by(() => {
  return sources.map(source => {
    let target = cache.get(source);
    if(!target) {
      target = new Target(source);
      cache.set(source, target);
    }
    return target;
  });
});

Reproduction

https://svelte.dev/playground/151a2151934544288c5c5a793a09c27a?version=5.0.5

Click "Add source" few times then "Mark all as existing".

All rows should render target: {id} exists=true", instead only latest one (which hasn't been reused in $derived cache) properly updates.

Logs

No response

System Info

System:
  OS: macOS 14.6.1
  CPU: (8) arm64 Apple M1
  Memory: 142.06 MB / 8.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 18.19.0 - ~/.nvm/versions/node/v18.19.0/bin/node
  Yarn: 1.22.22 - ~/.nvm/versions/node/v18.19.0/bin/yarn
  npm: 10.2.3 - ~/.nvm/versions/node/v18.19.0/bin/npm
  pnpm: 8.15.5 - ~/.nvm/versions/node/v18.19.0/bin/pnpm
  bun: 1.0.15 - ~/.bun/bin/bun
  Watchman: 2023.12.04.00 - /opt/homebrew/bin/watchman
Browsers:
  Chrome: 130.0.6723.69
  Safari: 17.6
npmPackages:
  svelte: ^5.0.5 => 5.0.5

Severity

blocking all usage of svelte

[trueadm]

This is expected behaviour. When you create a derived inside another derived, it is "owned" by that derived. So when the derived runs each time, it first removes any old deriveds that were created inside it, ready to attach the new ones.

That's because deriveds are meant to be pure functions without any side-effects. Given you've implemented a side-effect cache, it means that the derived is side-effectful and thus you're going to get odd results.

[ampatspell]

@trueadm, I'm coming from Ember.js, I didn't even consider that. Makes perfect sense, thank you for clarification!

I switched to $effect that mutates "target" arrray, everything works as expected.