fix

packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js

@@ -83,6 +83,10 @@ function build_assignment(operator, left, right, context) {
 		const path = context.path.map((node) => node.type);
 		const is_primitive = path.at(-1) === 'BindDirective' && path.at(-2) === 'RegularElement';
 
+		let value = /** @type {Expression} */ (
+			context.visit(build_assignment_value(operator, left, right))
+		);
+
 		if (
 			!is_primitive &&
 			binding.kind !== 'prop' &&
@@ -93,13 +97,9 @@ function build_assignment(operator, left, right, context) {
 			// other operators result in coercion
 			['=', '||=', '&&=', '??='].includes(operator)
 		) {
-			right = build_proxy_reassignment(right, object);
+			value = build_proxy_reassignment(value, object);
 		}
 
-		let value = /** @type {Expression} */ (
-			context.visit(build_assignment_value(operator, left, right))
-		);
-
 		return transform.assign(object, value);
 	}
 

packages/svelte/tests/runtime-runes/samples/proxy-nullish-coalescing-assignment/_config.js

@@ -2,24 +2,15 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
-	html: `<button>items: null</button><button>items: null</button>`,
+	html: `<button>items: null</button>`,
 
 	test({ assert, target }) {
 		const [btn1, btn2] = target.querySelectorAll('button');
 
 		flushSync(() => btn1.click());
-		assert.htmlEqual(target.innerHTML, `<button>items: [0]</button><button>items: null</button>`);
+		assert.htmlEqual(target.innerHTML, `<button>items: [0]</button>`);
 
 		flushSync(() => btn1.click());
-		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button><button>items: null</button>`);
-
-		flushSync(() => btn2.click());
-		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button><button>items: [0]</button>`);
-
-		flushSync(() => btn2.click());
-		assert.htmlEqual(
-			target.innerHTML,
-			`<button>items: [0,1]</button><button>items: [0,1]</button>`
-		);
+		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button>`);
 	}
 });

packages/svelte/tests/runtime-runes/samples/proxy-nullish-coalescing-assignment/main.svelte

@@ -1,25 +1,7 @@
 <script>
 	let items = $state(null);
-
-	class Foo {
-		#items = $state(null);
-
-		get items() {
-			return this.#items;
-		}
-
-		add() {
-			(this.#items ??= []).push(this.#items.length);
-		}
-	}
-
-	const foo = new Foo();
 </script>
 
 <button onclick={() => (items ??= []).push(items.length)}>
 	items: {JSON.stringify(items)}
 </button>
-
-<button onclick={() => foo.add()}>
-	items: {JSON.stringify(foo.items)}
-</button>