make autosubscribing to a nullish store a no-op (#4304)

* make autosubscribing to a nullish store a no-op (#2181)

* update changelog

* add test

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## 3.17.3
 
+* Make autosubscribing to a nullish store a no-op ([#2181](https://github.com/sveltejs/svelte/issues/2181))
 * Fix updating a `<slot>` inside an `{#if}` or other block ([#4292](https://github.com/sveltejs/svelte/issues/4292))
 * Fix using RxJS observables in `derived` stores ([#4298](https://github.com/sveltejs/svelte/issues/4298))
 * Add dev mode check to disallow duplicate keys in a keyed `{#each}` ([#4301](https://github.com/sveltejs/svelte/issues/4301))

src/runtime/internal/utils.ts

@@ -43,12 +43,15 @@ export function not_equal(a, b) {
 }
 
 export function validate_store(store, name) {
-	if (!store || typeof store.subscribe !== 'function') {
+	if (store != null && typeof store.subscribe !== 'function') {
 		throw new Error(`'${name}' is not a store with a 'subscribe' method`);
 	}
 }
 
 export function subscribe(store, ...callbacks) {
+	if (store == null) {
+		return noop;
+	}
 	const unsub = store.subscribe(...callbacks);
 	return unsub.unsubscribe ? () => unsub.unsubscribe() : unsub;
 }

test/runtime/samples/store-auto-subscribe-nullish/_config.js

@@ -0,0 +1,13 @@
+import { writable } from '../../../../store';
+
+export default {
+	html: `
+		<p>undefined</p>
+	`,
+	async test({ assert, component, target }) {
+		component.store = writable('foo');
+		assert.htmlEqual(target.innerHTML, `
+			<p>foo</p>
+		`);
+	}
+};

test/runtime/samples/store-auto-subscribe-nullish/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let store;
+</script>
+
+<p>{$store}</p>