From 8e4de9b14500e195ec7f084c8c32b1782b31c5c9 Mon Sep 17 00:00:00 2001
From: Nitin Sahu <ns0779687@gmail.com>
Date: Sat, 21 Mar 2026 18:03:45 +0530
Subject: [PATCH] fix: null out effect.b in destroy_effect to prevent memory
 leak (#17980)

## Fix: #17881

### Root Cause

When a dynamic component switches (e.g. `<Component />`), branch effects
are destroyed via `destroy_effect()`.

However, the `effect.b` (Boundary reference) field was not cleared
alongside other references (`next`, `prev`, `ctx`, `deps`, `fn`,
`nodes`, `ac`).

As a result, destroyed effects retained a reference to the `Boundary`
instance, which holds references to component state and child effects.
This prevented destroyed component subtrees from being garbage
collected, causing memory usage to grow during repeated component
switching.

### Solution

Clear the boundary reference during effect destruction.

```diff
effect.next =
      effect.prev =
      effect.teardown =
      effect.ctx =
      effect.deps =
      effect.fn =
      effect.nodes =
      effect.ac =
+     effect.b =
          null;
```

### Test Plan

* All existing tests pass:

  * runtime-runes: 2,459
  * runtime-legacy: 3,294
  * signals: 96
  * Total: 5,849 tests

* Added a dynamic component switching test that toggles components
repeatedly and verifies correct DOM output.

### Validation

Reproduced the issue using the example from #17881.

After applying the fix:

* Memory usage stabilizes during repeated component switching
* Destroyed components are properly reclaimed by the garbage collector
* No behavioral regressions observed

### Impact

* Fixes memory leak in dynamic component switching
* Minimal, safe change
* No API or behavior changes

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/fix-memory-leak-boundary.md                    | 5 +++++
 packages/svelte/src/internal/client/reactivity/effects.js | 1 +
 2 files changed, 6 insertions(+)
 create mode 100644 .changeset/fix-memory-leak-boundary.md

diff --git a/.changeset/fix-memory-leak-boundary.md b/.changeset/fix-memory-leak-boundary.md
new file mode 100644
index 0000000000..8a3b084981
--- /dev/null
+++ b/.changeset/fix-memory-leak-boundary.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: null out `effect.b` in `destroy_effect`
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index aeffeedddd..54c8a17d79 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -559,6 +559,7 @@ export function destroy_effect(effect, remove_dom = true) {
 		effect.fn =
 		effect.nodes =
 		effect.ac =
+		effect.b =
 			null;
 }