fix: allow $store to be used with changing values including nullish values (#7947)

fixes #7555
breaking change: derived now throws an error if you pass falsy values

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

CHANGELOG.md

@@ -10,9 +10,11 @@
 * **breaking** Stricter types for `onMount` - now throws a type error when returning a function asynchronously to catch potential mistakes around callback functions (see PR for migration instructions) ([#8136](https://github.com/sveltejs/svelte/pull/8136))
 * **breaking** Overhaul and drastically improve creating custom elements with Svelte (see PR for list of changes and migration instructions) ([#8457](https://github.com/sveltejs/svelte/pull/8457))
 * **breaking** Deprecate `SvelteComponentTyped`, use `SvelteComponent` instead ([#8512](https://github.com/sveltejs/svelte/pull/8512))
+* **breaking** Error on falsy values instead of stores passed to `derived` ([#7947](https://github.com/sveltejs/svelte/pull/7947))
 * Add `a11y no-noninteractive-element-interactions` rule ([#8391](https://github.com/sveltejs/svelte/pull/8391))
 * Add `a11y-no-static-element-interactions`rule ([#8251](https://github.com/sveltejs/svelte/pull/8251))
 * Bind `null` option and input values consistently ([#8312](https://github.com/sveltejs/svelte/issues/8312))
+* Allow `$store` to be used with changing values including nullish values ([#7555](https://github.com/sveltejs/svelte/issues/7555))
 
 ## Unreleased (3.0)
 

src/runtime/internal/utils.ts

@@ -68,6 +68,9 @@ export function validate_store(store, name) {
 
 export function subscribe(store, ...callbacks) {
 	if (store == null) {
+		for (const callback of callbacks) {
+			callback(undefined);
+		}
 		return noop;
 	}
 	const unsub = store.subscribe(...callbacks);

src/runtime/store/index.ts

@@ -164,9 +164,10 @@ export function derived<S extends Stores, T>(
 
 export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Readable<T> {
 	const single = !Array.isArray(stores);
-	const stores_array: Array<Readable<any>> = single
+	const stores_array: Array<Readable<any>> = single ? [stores as Readable<any>] : stores as Array<Readable<any>>;
-		? [stores as Readable<any>]
+	if (!stores_array.every(Boolean)) {
-		: stores as Array<Readable<any>>;
+		throw new Error('derived() expects stores as input, got a falsy value');
+	}
 
 	const auto = fn.length < 2;
 

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

@@ -0,0 +1,21 @@
+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>
+		`);
+		component.store = undefined;
+		assert.htmlEqual(target.innerHTML, `
+			<p>undefined</p>
+		`);
+		component.store = writable('bar');
+		assert.htmlEqual(target.innerHTML, `
+			<p>bar</p>
+		`);
+	}
+};

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

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

test/store/index.js

@@ -428,6 +428,21 @@ describe('store', () => {
 			a.set(false);
 			assert.equal(b_started, false);
 		});
+
+		it('errors on undefined stores #1', () => {
+			assert.throws(() => {
+				derived(null, (n) => n);
+			});
+		});
+
+		it('errors on undefined stores #2', () => {
+			assert.throws(() => {
+				const a = writable(1);
+				derived([a, null, undefined], ([n]) => {
+					return n * 2;
+				});
+			});
+		});
 	});
 
 	describe('get', () => {