fix: ensure no data loss occurs when using reactive Set methods

.changeset/honest-pans-kick.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure no data loss occurs when using reactive Set methods

packages/svelte/src/reactivity/set.js

@@ -51,18 +51,23 @@ export class ReactiveSet extends Set {
 			// @ts-ignore
 			proto[method] = function (...v) {
 				get(this.#version);
+				// We don't populate the underlying Set, so we need to create a clone using
+				// our internal values and then pass that to the method.
+				var clone = new Set(this.values());
 				// @ts-ignore
-				return set_proto[method].apply(this, v);
+				return set_proto[method].apply(clone, v);
 			};
 		}
 
 		for (const method of set_like_methods) {
 			// @ts-ignore
 			proto[method] = function (...v) {
 				get(this.#version);
-
+				// We don't populate the underlying Set, so we need to create a clone using
+				// our internal values and then pass that to the method.
+				var clone = new Set(this.values());
 				// @ts-ignore
-				var set = /** @type {Set<T>} */ (set_proto[method].apply(this, v));
+				var set = /** @type {Set<T>} */ (set_proto[method].apply(clone, v));
 				return new ReactiveSet(set);
 			};
 		}

packages/svelte/src/reactivity/set.test.ts

@@ -86,3 +86,23 @@ test('set.delete(...)', () => {
 
 	assert.deepEqual(Array.from(set.values()), [1, 2]);
 });
+
+test('set.forEach()', () => {
+	const set = new ReactiveSet([1, 2, 3, 4, 5]);
+
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			set.forEach((v) => log.push(v));
+		});
+	});
+
+	flushSync(() => {
+		set.add(6);
+	});
+
+	assert.deepEqual(log, [1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 6]);
+
+	cleanup();
+});