advanced type for derived

src/store.ts

@@ -10,24 +10,24 @@ type Invalidater<T> = (value?: T) => void;
 
 type StartStopNotifier<T> = (set: Subscriber<T>) => Unsubscriber | void;
 
-export interface ReadableStore<T> {
+export interface Readable<T> {
 	subscribe(run: Subscriber<T>, invalidate?: Invalidater<T>): Unsubscriber;
 }
 
-export interface WritableStore<T> extends ReadableStore<T> {
+export interface Writable<T> extends Readable<T> {
 	set(value: T): void;
 	update(updater: Updater<T>): void;
 }
 
 type SubscribeInvalidateTuple<T> = [Subscriber<T>, Invalidater<T>];
 
-export function readable<T>(value: T, start: StartStopNotifier<T>): ReadableStore<T> {
+export function readable<T>(value: T, start: StartStopNotifier<T>): Readable<T> {
 	return {
 		subscribe: writable(value, start).subscribe,
 	};
 }
 
-export function writable<T>(value: T, start: StartStopNotifier<T> = noop): WritableStore<T> {
+export function writable<T>(value: T, start: StartStopNotifier<T> = noop): Writable<T> {
 	let stop: Unsubscriber;
 	const subscribers: Array<SubscribeInvalidateTuple<T>> = [];
 
@@ -68,21 +68,27 @@ export function writable<T>(value: T, start: StartStopNotifier<T> = noop): Writa
 	return { set, update, subscribe };
 }
 
-export function derived<T>(
-	stores: ReadableStore<T> | Array<ReadableStore<T>>,
-	fn: (values: T | T[], set?: Subscriber<T>) => T | Unsubscriber | void,
-	initial_value: T): ReadableStore<T> {
+type Stores = Readable<any> | [Readable<any>, ...Array<Readable<any>>];
+
+type StoresValues<T> = T extends Readable<infer U> ? U :
+	{ [K in keyof T]: T[K] extends Readable<infer U> ? U : never };
+
+export function derived<T, S extends Stores>(
+	stores: S,
+	fn: (values: StoresValues<S>, set?: Subscriber<T>) => T | Unsubscriber | void,
+	initial_value?: T,
+): Readable<T> {
 
 	const single = !Array.isArray(stores);
-	const stores_array: Array<ReadableStore<T>> = single
-		? [stores as ReadableStore<T>]
-		: stores as Array<ReadableStore<T>>;
+	const stores_array: Array<Readable<any>> = single
+		? [stores as Readable<any>]
+		: stores as Array<Readable<any>>;
 
 	const auto = fn.length < 2;
 
 	return readable(initial_value, (set) => {
 		let inited = false;
-		const values: T[] = [];
+		const values: StoresValues<S> = [] as StoresValues<S>;
 
 		let pending = 0;
 		let cleanup = noop;
@@ -123,7 +129,7 @@ export function derived<T>(
 	});
 }
 
-export function get<T>(store: ReadableStore<T>): T {
+export function get<T>(store: Readable<T>): T {
 	let value: T | undefined;
 	store.subscribe((_: T) => value = _)();
 	return value as T;

test/store.ts

@@ -1,5 +1,5 @@
 import * as assert from 'assert';
-import { readable, writable, derived, get } from '../../store.js';
+import { readable, writable, derived, get } from '../store';
 
 describe('store', () => {
 	describe('writable', () => {
@@ -30,10 +30,10 @@ describe('store', () => {
 				return () => called -= 1;
 			});
 
-			const unsubscribe1 = store.subscribe(() => {});
+			const unsubscribe1 = store.subscribe(() => { });
 			assert.equal(called, 1);
 
-			const unsubscribe2 = store.subscribe(() => {});
+			const unsubscribe2 = store.subscribe(() => { });
 			assert.equal(called, 1);
 
 			unsubscribe1();
@@ -73,7 +73,7 @@ describe('store', () => {
 				set(0);
 
 				return () => {
-					tick = () => {};
+					tick = () => { };
 					running = false;
 				};
 			});
@@ -242,11 +242,29 @@ describe('store', () => {
 
 			assert.deepEqual(cleaned_up, [2, 3, 4]);
 		});
+
+		it('allows derived with different types', () => {
+			const a = writable('one');
+			const b = writable(1);
+			const c = derived([a, b], ([a, b]) => `${a} ${b}`);
+
+			const values: string[] = [];
+
+			const unsubscribe = c.subscribe(value => {
+				values.push(value);
+			});
+
+			a.set('two');
+			b.set(2);
+			assert.deepEqual(values, 'two 2');
+
+			unsubscribe();
+		});
 	});
 
 	describe('get', () => {
 		it('gets the current value of a store', () => {
-			const store = readable(42, () => {});
+			const store = readable(42, () => { });
 			assert.equal(get(store), 42);
 		});
 	});

tsconfig.json

@@ -7,8 +7,7 @@
         "allowJs": true,
         "lib": ["es5", "es6", "dom"],
         "importHelpers": true,
-        "moduleResolution": "node",
-        "strict": true
+        "moduleResolution": "node"
     },
     "include": [
         "src"