fix: handle deletions of state proxy properties (#13008)

.changeset/beige-lamps-ring.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: always return true from `deleteProperty` trap

.changeset/green-windows-tap.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle deletions of previously-unread state proxy properties

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) {
-				set(s, UNINITIALIZED);
+			if (s === undefined) {
+				if (prop in target) {
+					sources.set(prop, source(UNINITIALIZED));
 				}
-
-			if (exists) {
+			} else {
+				set(s, UNINITIALIZED);
 				update_version(version);
 			}
 
-			return exists;
+			return true;
 		},
 
 		get(target, prop, receiver) {

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;
+});