fix: better error message for global variable assignments

.changeset/slick-teeth-exist.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: better error message for global variable assignments

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js

@@ -22,7 +22,10 @@ export function validate_assignment(node, argument, context) {
 		const binding = context.state.scope.get(argument.name);
 
 		if (context.state.analysis.runes) {
-			if (binding?.node === context.state.analysis.props_id) {
+			if (
+				context.state.analysis.props_id != null &&
+				binding?.node === context.state.analysis.props_id
+			) {
 				e.constant_assignment(node, '$props.id()');
 			}
 

packages/svelte/tests/runtime-runes/samples/global-variable-assignment/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+export default test({
+	error: 'x is not defined',
+	async test() {}
+});

packages/svelte/tests/runtime-runes/samples/global-variable-assignment/foo.svelte.js

@@ -0,0 +1 @@
+x = 1;

packages/svelte/tests/runtime-runes/samples/global-variable-assignment/main.svelte

@@ -0,0 +1,3 @@
+<script>
+	import "./foo.svelte.js";
+</script>