[fix] set auto-subscription to undefined when update store to falsy value

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/js/samples/capture-inject-state/expected.js

@@ -12,7 +12,7 @@ import {
 	safe_not_equal,
 	set_data_dev,
 	space,
-	subscribe,
+	subscribe_dynamic_store,
 	text,
 	validate_slots,
 	validate_store
@@ -102,7 +102,7 @@ function instance($$self, $$props, $$invalidate) {
 
 	let $prop,
 		$$unsubscribe_prop = noop,
-		$$subscribe_prop = () => ($$unsubscribe_prop(), $$unsubscribe_prop = subscribe(prop, $$value => $$invalidate(2, $prop = $$value)), prop);
+		$$subscribe_prop = () => ($$unsubscribe_prop(), $$unsubscribe_prop = subscribe_dynamic_store(prop, $$value => $$invalidate(2, $prop = $$value)), prop);
 
 	$$self.$$.on_destroy.push(() => $$unsubscribe_prop());
 	let { $$slots: slots = {}, $$scope } = $$props;

test/js/samples/component-store-reassign-invalidate/expected.js

@@ -11,7 +11,7 @@ import {
 	safe_not_equal,
 	set_data,
 	space,
-	subscribe,
+	subscribe_dynamic_store,
 	text
 } from "svelte/internal";
 
@@ -62,7 +62,7 @@ function create_fragment(ctx) {
 function instance($$self, $$props, $$invalidate) {
 	let $foo,
 		$$unsubscribe_foo = noop,
-		$$subscribe_foo = () => ($$unsubscribe_foo(), $$unsubscribe_foo = subscribe(foo, $$value => $$invalidate(1, $foo = $$value)), foo);
+		$$subscribe_foo = () => ($$unsubscribe_foo(), $$unsubscribe_foo = subscribe_dynamic_store(foo, $$value => $$invalidate(1, $foo = $$value)), foo);
 
 	$$self.$$.on_destroy.push(() => $$unsubscribe_foo());
 	let foo = writable(0);

test/js/samples/reactive-class-optimized/expected.js

@@ -9,7 +9,7 @@ import {
 	noop,
 	safe_not_equal,
 	space,
-	subscribe,
+	subscribe_dynamic_store,
 	toggle_class
 } from "svelte/internal";
 
@@ -136,7 +136,7 @@ function instance($$self, $$props, $$invalidate) {
 
 	let $reactiveDeclaration,
 		$$unsubscribe_reactiveDeclaration = noop,
-		$$subscribe_reactiveDeclaration = () => ($$unsubscribe_reactiveDeclaration(), $$unsubscribe_reactiveDeclaration = subscribe(reactiveDeclaration, $$value => $$invalidate(3, $reactiveDeclaration = $$value)), reactiveDeclaration);
+		$$subscribe_reactiveDeclaration = () => ($$unsubscribe_reactiveDeclaration(), $$unsubscribe_reactiveDeclaration = subscribe_dynamic_store(reactiveDeclaration, $$value => $$invalidate(3, $reactiveDeclaration = $$value)), reactiveDeclaration);
 
 	component_subscribe($$self, reactiveStoreVal, $$value => $$invalidate(2, $reactiveStoreVal = $$value));
 	$$self.$$.on_destroy.push(() => $$unsubscribe_reactiveDeclaration());

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}