throw, changelog

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/store/index.ts

@@ -164,10 +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>>;
-		// Fall back to readable() for falsy stores in array, else derived store will be forever pending
+	if (!stores_array.every(Boolean)) {
-		? [(stores || readable()) as Readable<any>]
+		throw new Error('derived() expects stores as input, got a falsy value')
-		: (stores as Array<Readable<any>>).map(store => store || readable());
+	}
 
 	const auto = fn.length < 2;
 

test/store/index.js

@@ -429,37 +429,19 @@ describe('store', () => {
 			assert.equal(b_started, false);
 		});
 
-		it('works with undefined stores #1', () => {
+		it('errors on undefined stores #1', () => {
-			const a = derived(null, (n) => {
+			assert.throws(() => {
-				return n;
+				derived(null, (n) => n);
 			});
-			const values = [];
-			const unsubscribe = a.subscribe((value) => values.push(value));
-			unsubscribe();
-			assert.deepEqual(values, [undefined]);
 		});
 
-		it('works with undefined stores #2', () => {
+		it('errors on undefined stores #2', () => {
-			const a = writable(1);
+			assert.throws(() => {
-			const b = derived([a, null, undefined], ([n, un1, un2]) => {
+				const a = writable(1);
-				assert.equal(un1, undefined);
+				derived([a, null, undefined], ([n]) => {
-				assert.equal(un2, undefined);
+					return n * 2;
-				return n * 2;
+				});
-			});
-
-			const values = [];
-
-			const unsubscribe = b.subscribe(value => {
-				values.push(value);
 			});
-
-			a.set(2);
-			assert.deepEqual(values, [2, 4]);
-
-			unsubscribe();
-
-			a.set(3);
-			assert.deepEqual(values, [2, 4]);
 		});
 	});