Svelte Reactivity Bug with `$derived` in class

[timephy]

Describe the bug

I used a "getter", a$derived and a $state together inside a class.

This worked before, but after updating to the latest version it stopped being reliable.

I noticed that reactivity to the below mentioned get layout() does not work reliably.
(it does start working in some conditions I could not understand)

---

→ TLDR: Using $derived inside a class is unreliable, using a "getter" instead is not.

This version created a reactivity problem. Where even embedding the text {new Layout().layout} would not be reactive to changes of #orientation.

class Layout {
  #orientation: OrientationType = $state("portrait-primary")
  #orientationLock: OrientationType | null = $state(null)
  #orientationEffective = $derived(this.#orientationLock ?? this.#orientation)
  #isHorizontal = $derived(this.#orientationEffective.startsWith("landscape"))

  // this private state is then exported using a getter

  get layout(): LayoutType {
        return this.#isLargeWidth
            ? "with-sidebar"
            : this.#isHorizontal && PLATFORM === "mobile"
              ? "with-sidebar-tiny"
              : "only-main"
  }
}

→ This problem was solved when changing the $derived variables into getters themselves:

// ...
    get #orientationEffective() {
        return this.#orientationLock ?? this.#orientation
    }
    get #isHorizontal() {
        return this.#orientationEffective.startsWith("landscape")
    }
// ...

Additional context: I use the resulting state inside a snippet and pass this snippet to another component to render it.

{#snippet snip()}
    <PlayerTab
        layout={LAYOUT.layout === "with-sidebar-tiny" ? "landscape" : "normal"}
    />
{/snippet}

Observing the state from the parent is NOT enough to fix the bug:

{LAYOUT.layout} 
{#snippet snip()}
    {LAYOUT.layout}
    <PlayerTab
        layout={LAYOUT.layout === "with-sidebar-tiny" ? "landscape" : "normal"}
    />
{/snippet}

HOWEVER observing it from inside <PlayerTab /> ({LAYOUT.layout}) IS ENOUGH to fix it.
Observing only the layout prop (as text: {layout}) is however not enough... How weird.
LAYOUT.layout and layout are both used in multiple {#if} blocks/etc inside PlayerTab.

Reproduction

I am very sorry to say that I was not able to reproduce this in the playground (tried for around 30 minutes)

→ However the described bug is 100% reliable, and also 100% fixable by changing $deriveds into "getter"s!

→ (It also seems to be related to the use of null ?? "fallback" inside $derived...?)

Logs

System Info

System:
    OS: macOS 15.4.1
    CPU: (20) x64 Intel(R) Xeon(R) W-2150B CPU @ 3.00GHz
    Memory: 205.15 MB / 64.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 23.11.0 - /usr/local/bin/node
    Yarn: 1.22.11 - /usr/local/bin/yarn
    npm: 10.9.2 - /usr/local/bin/npm
    pnpm: 10.7.1 - /usr/local/bin/pnpm
    bun: 1.0.25 - /usr/local/bin/bun
  Browsers:
    Chrome: 136.0.7103.48
    Safari: 18.4
  npmPackages:
    svelte: 5.28.2 => 5.28.2

Severity

blocking an upgrade

[timephy]

Okay, I managed to create a (similar) reproduction.
https://svelte.dev/playground/65217d0486a04acf9c5f2c8275fe63b8?version=5.28.2

• It only causes the bug when commenting in+out the observation <p>{CLS.countP2}</p> in App.svelte
• It is NOT fixed by changing $derived to a "getter"
• Observing layout inside Comp.svelte is NOT enough to fix it.

This behaviour differs from the bug described before. Maybe these are not even related.....

Because of this obscure behaviour, please see this screen recording.
And see that after commenting the observation for the second time, the reactivity breaks.

Screen.Recording.2025-04-30.at.13.19.20.mov

[timephy]

Might be related to #15829 and #15832.

[hermit99]

Having the same problem with Svelte 5.28.2. The $derived in class is not reactive.

My work-around for now is to add an $inspect(classA_instance.derivedX) in the component's script, then it works.

[7nik]

@hermit99 $inspect is dev-only and doesn't exist in prod.

[hermit99]

@timephy @7nik To help you guys diagnose, I took some time to create a simplified repo: https://svelte.dev/playground/a3ff881bd3bb4011b9c3044ea22a876b?version=5.28.2

Notice the template is not updated. However if we comment out line 5 in main: ui.errors.e1 = null then everything is working again.

Can also confirm it's working fine on Svelte@5.28.1: https://svelte.dev/playground/a3ff881bd3bb4011b9c3044ea22a876b?version=5.28.1

From changes between 5.28.1 and 5.28.2, I doubt commit d0dcc0b or bfb969a are most likely the cause?

[7nik]

Looking at the REPL, I'm quite sure it is #15829 where the cause are deriveds created outside the reactive context + re-calculating them into the save value as previous one.

[timephy]

Hey,

• This issue is breaking reactivity
• A main feature of Svelte
• With something as common as using a standard TypeScript feature || and ??

Could someone please pay a little attention to this and fix it?