From f21163a5ae63c3f928b73a82e2281a5a4c064964 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Mon, 15 Jul 2024 09:34:52 +0200
Subject: [PATCH] fix: ensure `$state.snapshot` never errors

Snapshotting can error on un-cloneable objects. It's not practical to error in this case; often there's no way out of this for users, so it makes sense to return the original value in that case, and warn in dev mode about it.

closes #12438
---
 .changeset/brave-gorillas-fold.md               |  5 +++++
 .../svelte/messages/shared-warnings/warnings.md |  4 ++++
 packages/svelte/src/internal/shared/clone.js    | 17 ++++++++++++++++-
 packages/svelte/src/internal/shared/warnings.js | 12 ++++++++++++
 4 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/brave-gorillas-fold.md

diff --git a/.changeset/brave-gorillas-fold.md b/.changeset/brave-gorillas-fold.md
new file mode 100644
index 0000000000..5a551cbff2
--- /dev/null
+++ b/.changeset/brave-gorillas-fold.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure `$state.snapshot` never errors
diff --git a/packages/svelte/messages/shared-warnings/warnings.md b/packages/svelte/messages/shared-warnings/warnings.md
index 8154436843..ee5b32c9f4 100644
--- a/packages/svelte/messages/shared-warnings/warnings.md
+++ b/packages/svelte/messages/shared-warnings/warnings.md
@@ -1,3 +1,7 @@
 ## dynamic_void_element_content
 
 > `<svelte:element this="%tag%">` is a void element — it cannot have content
+
+## state_snapshot_uncloneable
+
+> An object could not be cloned with $state.snapshot, the original value will be returned
diff --git a/packages/svelte/src/internal/shared/clone.js b/packages/svelte/src/internal/shared/clone.js
index 715ec3d855..51dee7da47 100644
--- a/packages/svelte/src/internal/shared/clone.js
+++ b/packages/svelte/src/internal/shared/clone.js
@@ -1,4 +1,6 @@
 /** @import { Snapshot } from './types' */
+import { DEV } from 'esm-env';
+import * as w from './warnings.js';
 import { get_prototype_of, is_array, object_prototype } from './utils.js';
 
 /**
@@ -50,5 +52,18 @@ function clone(value, cloned) {
 		}
 	}
 
-	return /** @type {Snapshot<T>} */ (structuredClone(value));
+	if (value instanceof EventTarget) {
+		// can't be cloned
+		return /** @type {Snapshot<T>} */ (value);
+	}
+
+	try {
+		return /** @type {Snapshot<T>} */ (structuredClone(value));
+	} catch (e) {
+		if (DEV) {
+			w.state_snapshot_uncloneable();
+			console.warn(e);
+		}
+		return /** @type {Snapshot<T>} */ (value);
+	}
 }
diff --git a/packages/svelte/src/internal/shared/warnings.js b/packages/svelte/src/internal/shared/warnings.js
index 2fc1b6fdd2..bef3b05c4f 100644
--- a/packages/svelte/src/internal/shared/warnings.js
+++ b/packages/svelte/src/internal/shared/warnings.js
@@ -16,4 +16,16 @@ export function dynamic_void_element_content(tag) {
 		// TODO print a link to the documentation
 		console.warn("dynamic_void_element_content");
 	}
+}
+
+/**
+ * An object could not be cloned with $state.snapshot, the original value will be returned
+ */
+export function state_snapshot_uncloneable() {
+	if (DEV) {
+		console.warn(`%c[svelte] state_snapshot_uncloneable\n%cAn object could not be cloned with $state.snapshot, the original value will be returned`, bold, normal);
+	} else {
+		// TODO print a link to the documentation
+		console.warn("state_snapshot_uncloneable");
+	}
 }
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