From e705369dee060e74ad9456e45824f36b1ee3c6cf Mon Sep 17 00:00:00 2001
From: Elliott Johnson <hello@ell.iott.dev>
Date: Wed, 27 May 2026 12:41:35 -0600
Subject: [PATCH] fix: propagate async @const blockers through closure
 references (#18309)

Right now, if you have the following:

```svelte
{@const data = await foo}
<p>{(() => data)()}<p>
```

It will blow up during CSR, because it tries to read `data` before it
exists. The solution to this is to consider references inside closures
as blockers for those closures. This _does_ mean we'll overblock in some
circumstances, such as:

```svelte
{@const data = await foo}
<button onclick={() => data}>foo<button>
```

But I wonder if that's actually incorrect? If the user were to click
`onclick` before `data` is ready, it would blow up.

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
---
 .changeset/iife-await-const-blocker.md             |  5 +++++
 .../3-transform/client/visitors/shared/utils.js    |  2 +-
 packages/svelte/src/compiler/phases/nodes.js       |  4 ++--
 .../samples/async-each-const-await-iife/_config.js | 14 ++++++++++++++
 .../async-each-const-await-iife/main.svelte        | 10 ++++++++++
 5 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 .changeset/iife-await-const-blocker.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/main.svelte

diff --git a/.changeset/iife-await-const-blocker.md b/.changeset/iife-await-const-blocker.md
new file mode 100644
index 0000000000..2b5c4f0323
--- /dev/null
+++ b/.changeset/iife-await-const-blocker.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: propagate async `@const` blockers through closure references so template expressions like `{(() => host)()}` correctly wait for the awaited value
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
index 1f3c7b2256..51c84dd01c 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js
@@ -52,7 +52,7 @@ export class Memoizer {
 	 * @param {ExpressionMetadata} metadata
 	 */
 	check_blockers(metadata) {
-		for (const binding of metadata.dependencies) {
+		for (const binding of metadata.references) {
 			if (binding.blocker) {
 				this.#blockers.add(binding.blocker);
 			}
diff --git a/packages/svelte/src/compiler/phases/nodes.js b/packages/svelte/src/compiler/phases/nodes.js
index 2e54f333a8..48ebacb50b 100644
--- a/packages/svelte/src/compiler/phases/nodes.js
+++ b/packages/svelte/src/compiler/phases/nodes.js
@@ -102,8 +102,8 @@ export class ExpressionMetadata {
 		if (!this.#blockers) {
 			this.#blockers = new Set();
 
-			for (const d of this.dependencies) {
-				if (d.blocker) this.#blockers.add(d.blocker);
+			for (const r of this.references) {
+				if (r.blocker) this.#blockers.add(r.blocker);
 			}
 		}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/_config.js b/packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/_config.js
new file mode 100644
index 0000000000..958c3023f1
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/_config.js
@@ -0,0 +1,14 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Tests that an IIFE referencing an `await` @const inside an {#each} block
+// correctly registers the async dependency so the template waits for the
+// resolved value instead of rendering with `undefined`.
+export default test({
+	mode: ['client', 'hydrate', 'async-server'],
+	ssrHtml: '<p>example.com</p>',
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>example.com</p>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/main.svelte
new file mode 100644
index 0000000000..e0ccbb7892
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-each-const-await-iife/main.svelte
@@ -0,0 +1,10 @@
+<script>
+	async function get_host() {
+		return 'example.com';
+	}
+</script>
+
+{#each { length: 1 } as nothing}
+	{@const host = await get_host()}
+	<p>{(() => host)()}</p>
+{/each}