From cb4f82f811630c0ade5c5498306de5d1647b8b83 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sat, 24 Aug 2024 17:37:42 -0400
Subject: [PATCH] fix: handle deletions of state proxy properties (#13008)

---
 .changeset/beige-lamps-ring.md                    |  5 +++++
 .changeset/green-windows-tap.md                   |  5 +++++
 packages/svelte/src/internal/client/proxy.js      | 14 +++++++-------
 packages/svelte/src/internal/client/proxy.test.ts | 11 +++++++++++
 4 files changed, 28 insertions(+), 7 deletions(-)
 create mode 100644 .changeset/beige-lamps-ring.md
 create mode 100644 .changeset/green-windows-tap.md

diff --git a/.changeset/beige-lamps-ring.md b/.changeset/beige-lamps-ring.md
new file mode 100644
index 0000000000..f647921060
--- /dev/null
+++ b/.changeset/beige-lamps-ring.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: always return true from `deleteProperty` trap
diff --git a/.changeset/green-windows-tap.md b/.changeset/green-windows-tap.md
new file mode 100644
index 0000000000..964d6373bc
--- /dev/null
+++ b/.changeset/green-windows-tap.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle deletions of previously-unread state proxy properties
diff --git a/packages/svelte/src/internal/client/proxy.js b/packages/svelte/src/internal/client/proxy.js
index 333cd20b58..2a81a3cd53 100644
--- a/packages/svelte/src/internal/client/proxy.js
+++ b/packages/svelte/src/internal/client/proxy.js
@@ -19,7 +19,7 @@ import * as e from './errors.js';
  * @param {T} value
  * @param {ProxyMetadata | null} [parent]
  * @param {Source<T>} [prev] dev mode only
- * @returns {ProxyStateObject<T> | T}
+ * @returns {T}
  */
 export function proxy(value, parent = null, prev) {
 	// if non-proxyable, or is already a proxy, return `value`
@@ -91,17 +91,17 @@ export function proxy(value, parent = null, prev) {
 
 		deleteProperty(target, prop) {
 			var s = sources.get(prop);
-			var exists = s !== undefined ? s.v !== UNINITIALIZED : prop in target;
 
-			if (s !== undefined) {
+			if (s === undefined) {
+				if (prop in target) {
+					sources.set(prop, source(UNINITIALIZED));
+				}
+			} else {
 				set(s, UNINITIALIZED);
-			}
-
-			if (exists) {
 				update_version(version);
 			}
 
-			return exists;
+			return true;
 		},
 
 		get(target, prop, receiver) {
diff --git a/packages/svelte/src/internal/client/proxy.test.ts b/packages/svelte/src/internal/client/proxy.test.ts
index d73e6bef72..d15e656d83 100644
--- a/packages/svelte/src/internal/client/proxy.test.ts
+++ b/packages/svelte/src/internal/client/proxy.test.ts
@@ -85,3 +85,14 @@ test('does not re-proxy proxies', () => {
 	assert.equal(inner.count, 1);
 	assert.equal(outer.inner.count, 1);
 });
+
+test('deletes a property', () => {
+	const state = proxy({ a: 1, b: 2 } as { a?: number; b?: number; c?: number });
+
+	delete state.a;
+	assert.equal(JSON.stringify(state), '{"b":2}');
+	delete state.a;
+
+	// deleting a non-existent property should succeed
+	delete state.c;
+});