fix: ensure transient writes to tracked parent effects works as expected (#15506)

trueadm · Rich-Harris · web-flow · commit 5d3aa2bda4be · 2025-03-13T19:49:26.000Z
* ix: ensure transient writes to tracked parent effects works as expected

* lint

* format test

* tweak changeset

---------

Co-authored-by: Rich Harris &lt;rich.harris@vercel.com&gt;
diff --git a/.changeset/brown-rockets-shake.md b/.changeset/brown-rockets-shake.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: invalidate parent effects when child effects update parent dependencies
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
@@ -460,6 +460,14 @@ export function update_reaction(reaction) {
 		// the same version
 		if (previous_reaction !== null) {
 			read_version++;
+
+			if (untracked_writes !== null) {
+				if (previous_untracked_writes === null) {
+					previous_untracked_writes = untracked_writes;
+				} else {
+					previous_untracked_writes.push(.../** @type {Source[]} */ (untracked_writes));
+				}
+			}
 		}
 
 		return result;
diff --git a/packages/svelte/tests/runtime-runes/samples/untracked-write-pre/_config.js b/packages/svelte/tests/runtime-runes/samples/untracked-write-pre/_config.js
@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, logs }) {
+		assert.deepEqual(logs, ['Outer', 'Inner', 'Outer', 'Inner']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/untracked-write-pre/main.svelte b/packages/svelte/tests/runtime-runes/samples/untracked-write-pre/main.svelte
@@ -0,0 +1,13 @@
+<script>
+	let value = $state(0);
+
+	$effect.pre(() => {
+		console.log("Outer");
+		value;
+
+		$effect.pre(() => {
+			console.log("Inner");
+			value = 10;
+		});
+	});
+</script>

-Original file line number
+Diff line change
@@ @@ -0,0 +1,5 @@ @@
 +---
 +'svelte': patch
 +---
++
 +fix: invalidate parent effects when child effects update parent dependencies