fix: properly invoke iterator.return() during reactivity loss check (#17966)

dummdidumm · web-flow · commit 803c565febd0 · 2026-03-20T08:37:21.000+01:00
The logic was flipped - `iterator.return()` should be called when the iterator is abnormally ending, not when it's normally ending. Fixes #16610
diff --git a/.changeset/tasty-carrots-tie.md b/.changeset/tasty-carrots-tie.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: properly invoke `iterator.return()` during reactivity loss check
diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
@@ -209,8 +209,8 @@ export async function* for_await_track_reactivity_loss(iterable) {
 			yield value;
 		}
 	} finally {
-		// If the iterator had a normal completion and `return` is defined on the iterator, call it and return the value
-		if (normal_completion && iterator.return !== undefined) {
+		// If the iterator had an abrupt completion and `return` is defined on the iterator, call it and return the value
+		if (!normal_completion && iterator.return !== undefined) {
 			// eslint-disable-next-line no-unsafe-finally
 			return /** @type {TReturn} */ ((await track_reactivity_loss(iterator.return()))().value);
 		}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-reactivity-loss-for-await-break-return/_config.js b/packages/svelte/tests/runtime-runes/samples/async-reactivity-loss-for-await-break-return/_config.js
@@ -0,0 +1,21 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+import { normalise_trace_logs } from '../../../helpers.js';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	html: '<p>pending</p>',
+	async test({ assert, target, warnings }) {
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<h1>number -> number -> number -> return -> ended</h1>');
+
+		assert.deepEqual(normalise_trace_logs(warnings), [
+			{
+				log: 'Detected reactivity loss when reading `values.length`. This happens when state is read in an async function after an earlier `await`'
+			}
+		]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-reactivity-loss-for-await-break-return/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-reactivity-loss-for-await-break-return/main.svelte
@@ -0,0 +1,41 @@
+<script>
+	let values = $state([0, 1, 2]);
+
+	async function get_result() {
+		const logs = [];
+
+		const iterator = {
+			index: 0,
+			async next() {
+				if (this.index >= values.length) return { done: true };
+				return { done: false, value: values[this.index++] };
+			},
+			async return() {
+				logs.push('return');
+				return { done: true };
+			},
+			[Symbol.asyncIterator]() {
+				return this;
+			}
+		};
+
+		for await (const value of iterator) {
+			logs.push('number');
+			// read reactive state after async iterator await
+			if (values.length === 3 && value === 2) {
+				break;
+			}
+		}
+
+		logs.push('ended');
+		return logs.join(' -> ');
+	}
+</script>
+
+<svelte:boundary>
+	<h1>{await get_result()}</h1>
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>

-Original file line number
+Diff line change
@@ @@ -0,0 +1,5 @@ @@
 +---
 +'svelte': patch
 +---
++
 +fix: properly invoke `iterator.return()` during reactivity loss check
Original file line number	Diff line number	Diff line change
`@@ -209,8 +209,8 @@ export async function* for_await_track_reactivity_loss(iterable) {`
`209`	`209`	`yield value;`
`210`	`210`	`}`
`211`	`211`	`} finally {`
`212`		- // If the iterator had a normal completion and `return` is defined on the iterator, call it and return the value
`213`		`- if (normal_completion && iterator.return !== undefined) {`
	`212`	+ // If the iterator had an abrupt completion and `return` is defined on the iterator, call it and return the value
	`213`	`+ if (!normal_completion && iterator.return !== undefined) {`
`214`	`214`	`// eslint-disable-next-line no-unsafe-finally`
`215`	`215`	`return /** @type {TReturn} */ ((await track_reactivity_loss(iterator.return()))().value);`
`216`	`216`	`}`