From 76cb89c75000b384946adfea39cfe83e47d1bd8f Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <trueadm@users.noreply.github.com>
Date: Mon, 5 Aug 2024 16:26:33 +0100
Subject: [PATCH] fix: allow deletion of $$restProps properties (#12736)

---
 .changeset/loud-news-deliver.md                   |  5 +++++
 .../src/internal/client/reactivity/props.js       |  7 +++++++
 .../samples/rest-props-delete/App.svelte          | 10 ++++++++++
 .../samples/rest-props-delete/_config.js          | 15 +++++++++++++++
 .../samples/rest-props-delete/main.svelte         |  5 +++++
 5 files changed, 42 insertions(+)
 create mode 100644 .changeset/loud-news-deliver.md
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/rest-props-delete/App.svelte
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/rest-props-delete/_config.js
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/rest-props-delete/main.svelte

diff --git a/.changeset/loud-news-deliver.md b/.changeset/loud-news-deliver.md
new file mode 100644
index 0000000000..f562caff8f
--- /dev/null
+++ b/.changeset/loud-news-deliver.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow deletion of $$restProps properties
diff --git a/packages/svelte/src/internal/client/reactivity/props.js b/packages/svelte/src/internal/client/reactivity/props.js
index b822f8d4ec..81c3e4d743 100644
--- a/packages/svelte/src/internal/client/reactivity/props.js
+++ b/packages/svelte/src/internal/client/reactivity/props.js
@@ -126,6 +126,13 @@ const legacy_rest_props_handler = {
 			};
 		}
 	},
+	deleteProperty(target, key) {
+		// Svelte 4 allowed for deletions on $$restProps
+		if (target.exclude.includes(key)) return false;
+		target.exclude.push(key);
+		update(target.version);
+		return true;
+	},
 	has(target, key) {
 		if (target.exclude.includes(key)) return false;
 		return key in target.props;
diff --git a/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/App.svelte b/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/App.svelte
new file mode 100644
index 0000000000..37825576e5
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/App.svelte
@@ -0,0 +1,10 @@
+<script>
+	function prune() {
+		$$restProps.a
+		delete $$restProps.a
+	}
+</script>
+
+<button on:click={prune}>delete a</button>
+
+{JSON.stringify($$restProps)}
diff --git a/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/_config.js b/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/_config.js
new file mode 100644
index 0000000000..853af731f9
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/_config.js
@@ -0,0 +1,15 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>delete a</button>\n{"a":1,"b":2}`,
+
+	test({ assert, target }) {
+		const [btn1] = target.querySelectorAll('button');
+
+		btn1.click();
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, `<button>delete a</button>\n{"b":2}`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/main.svelte b/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/main.svelte
new file mode 100644
index 0000000000..461848a3b3
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/rest-props-delete/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	import App from "./App.svelte";
+</script>
+
+<App a={1} b={2} />