set auto-subscription to undefined when update store to falsy value

Author: tanhauhau

src/compiler/compile/render_dom/index.ts

@@ -434,7 +434,7 @@ export default function dom(
 				const subscribe = `$$subscribe_${name}`;
 				const i = renderer.context_lookup.get($name).index;
 
-				return b`let ${$name}, ${unsubscribe} = @noop, ${subscribe} = () => (${unsubscribe}(), ${unsubscribe} = @subscribe(${name}, $$value => $$invalidate(${i}, ${$name} = $$value)), ${name})`;
+				return b`let ${$name}, ${unsubscribe} = @noop, ${subscribe} = () => (${unsubscribe}(), ${unsubscribe} = @subscribe_dynamic_store(${name}, $$value => $$invalidate(${i}, ${$name} = $$value)), ${name})`;
 			}
 
 			return b`let ${$name};`;

src/runtime/internal/utils.ts

@@ -72,6 +72,15 @@ export function subscribe(store, ...callbacks) {
 	return unsub.unsubscribe ? () => unsub.unsubscribe() : unsub;
 }
 
+export function subscribe_dynamic_store(store, callback) {
+	if (store == null) {
+		callback(undefined);
+		return noop;
+	}
+	const unsub = store.subscribe(callback);
+	return unsub.unsubscribe ? () => unsub.unsubscribe() : unsub;
+}
+
 export function get_store_value<T>(store: Readable<T>): T {
 	let value;
 	subscribe(store, _ => value = _)();

test/runtime/samples/store-resubscribe-d/_config.js

@@ -0,0 +1,13 @@
+import { writable } from 'svelte/store';
+
+export default {
+	html: '31',
+	async test({ assert, component, target, window }) {
+		component.store = undefined;
+
+		assert.htmlEqual(target.innerHTML, 'undefined');
+		
+		component.store = writable(42);
+		assert.htmlEqual(target.innerHTML, '42');
+	}
+};

test/runtime/samples/store-resubscribe-d/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	import { writable } from 'svelte/store';
+	export let store = writable(31);
+</script>
+
+{$store}