handle non-falsy non-function deriver return values - fixes #2780

src/store.ts

@@ -1,4 +1,4 @@
-import { run_all, noop, safe_not_equal } from './internal/utils';
+import { run_all, noop, safe_not_equal, is_function } from './internal/utils';
 
 type Subscriber<T> = (value: T) => void;
 
@@ -102,7 +102,7 @@ export function derived<T, S extends Stores>(
 			if (auto) {
 				set(result as T);
 			} else {
-				cleanup = result as Unsubscriber || noop;
+				cleanup = is_function(result) ? result as Unsubscriber : noop;
 			}
 		};
 

test/store.ts → test/store/index.ts

@@ -1,7 +1,7 @@
 import * as assert from 'assert';
-import { readable, writable, derived, get } from '../store';
+import { readable, writable, derived, get } from '../../store';
 
-describe('store', () => {
+describe.only('store', () => {
 	describe('writable', () => {
 		it('creates a writable store', () => {
 			const count = writable(0);
@@ -243,6 +243,30 @@ describe('store', () => {
 			assert.deepEqual(cleaned_up, [2, 3, 4]);
 		});
 
+		it('discards non-function return values', () => {
+			const num = writable(1);
+
+			const values = [];
+
+			const d = derived(num, ($num, set) => {
+				set($num * 2);
+				return {};
+			});
+
+			num.set(2);
+
+			const unsubscribe = d.subscribe(value => {
+				values.push(value);
+			});
+
+			num.set(3);
+			num.set(4);
+
+			assert.deepEqual(values, [4, 6, 8]);
+
+			unsubscribe();
+		});
+
 		it('allows derived with different types', () => {
 			const a = writable('one');
 			const b = writable(1);

test/test.js

@@ -2,6 +2,6 @@ const glob = require("tiny-glob/sync.js");
 
 require("./setup");
 
-glob("*/index.js", { cwd: "test" }).forEach(function(file) {
+glob("*/index.{js,ts}", { cwd: "test" }).forEach(function(file) {
 	require("./" + file);
 });