diff --git a/.gitignore b/.gitignore
index 06671edc2bf1..7aa75b29f44c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,7 +6,7 @@ node_modules
 /compiler.js
 /index.js
 /internal.*
-/store.js
+/store.*
 /easing.js
 /motion.*
 /transition.js
diff --git a/package.json b/package.json
index 437eef8d0e27..bbe8f5389879 100644
--- a/package.json
+++ b/package.json
@@ -27,11 +27,12 @@
     "precodecov": "npm run coverage",
     "lint": "eslint src test/*.js",
     "build": "rollup -c",
-    "prepare": "npm run build",
+    "prepare": "npm run build && npm run tsd",
     "dev": "rollup -cw",
     "pretest": "npm run build",
     "posttest": "agadoo src/internal/index.js",
-    "prepublishOnly": "export PUBLISH=true && npm run lint && npm test"
+    "prepublishOnly": "export PUBLISH=true && npm run lint && npm test",
+    "tsd": "tsc -d src/store.ts --outDir ."
   },
   "repository": {
     "type": "git",
diff --git a/rollup.config.js b/rollup.config.js
index f7b2d07d4bed..0d19e59d4ade 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -87,8 +87,37 @@ export default [
 		external: id => id.startsWith('svelte/')
 	},
 
+	/* store.mjs */
+	{
+		input: `src/store.ts`,
+		output: [
+			{
+				file: `store.mjs`,
+				format: 'esm',
+				paths: id => id.startsWith('svelte/') && id.replace('svelte', '.')
+			},
+			{
+				file: `store.js`,
+				format: 'cjs',
+				paths: id => id.startsWith('svelte/') && id.replace('svelte', '.')
+			}
+		],
+		plugins: [
+			is_publish
+				? typescript({
+					include: 'src/**',
+					exclude: 'src/internal/**',
+					typescript: require('typescript')
+				})
+				: sucrase({
+					transforms: ['typescript']
+				})
+		],
+		external: id => id.startsWith('svelte/')
+	},
+
 	// everything else
-	...['index', 'store', 'easing', 'transition', 'animate'].map(name => ({
+	...['index', 'easing', 'transition', 'animate'].map(name => ({
 		input: `${name}.mjs`,
 		output: {
 			file: `${name}.js`,
diff --git a/src/store.ts b/src/store.ts
new file mode 100644
index 000000000000..462f155c2ada
--- /dev/null
+++ b/src/store.ts
@@ -0,0 +1,136 @@
+import { run_all, noop, safe_not_equal } from './internal/utils';
+
+type Subscriber<T> = (value: T) => void;
+
+type Unsubscriber = () => void;
+
+type Updater<T> = (value: T) => T;
+
+type Invalidater<T> = (value?: T) => void;
+
+type StartStopNotifier<T> = (set: Subscriber<T>) => Unsubscriber | void;
+
+export interface Readable<T> {
+	subscribe(run: Subscriber<T>, invalidate?: Invalidater<T>): Unsubscriber;
+}
+
+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>): Readable<T> {
+	return {
+		subscribe: writable(value, start).subscribe,
+	};
+}
+
+export function writable<T>(value: T, start: StartStopNotifier<T> = noop): Writable<T> {
+	let stop: Unsubscriber;
+	const subscribers: Array<SubscribeInvalidateTuple<T>> = [];
+
+	function set(new_value: T): void {
+		if (safe_not_equal(value, new_value)) {
+			value = new_value;
+			if (!stop) {
+				return; // not ready
+			}
+			subscribers.forEach((s) => s[1]());
+			subscribers.forEach((s) => s[0](value));
+		}
+	}
+
+	function update(fn: Updater<T>): void {
+		set(fn(value));
+	}
+
+	function subscribe(run: Subscriber<T>, invalidate: Invalidater<T> = noop): Unsubscriber {
+		const subscriber: SubscribeInvalidateTuple<T> = [run, invalidate];
+		subscribers.push(subscriber);
+		if (subscribers.length === 1) {
+			stop = start(set) || noop;
+		}
+		run(value);
+
+		return () => {
+			const index = subscribers.indexOf(subscriber);
+			if (index !== -1) {
+				subscribers.splice(index, 1);
+			}
+			if (subscribers.length === 0) {
+				stop();
+			}
+		};
+	}
+
+	return { set, update, subscribe };
+}
+
+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<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: StoresValues<S> = [] as StoresValues<S>;
+
+		let pending = 0;
+		let cleanup = noop;
+
+		const sync = () => {
+			if (pending) {
+				return;
+			}
+			cleanup();
+			const result = fn(single ? values[0] : values, set);
+			if (auto) {
+				set(result as T);
+			} else {
+				cleanup = result as Unsubscriber || noop;
+			}
+		};
+
+		const unsubscribers = stores_array.map((store, i) => store.subscribe(
+			(value) => {
+				values[i] = value;
+				pending &= ~(1 << i);
+				if (inited) {
+					sync();
+				}
+			},
+			() => {
+				pending |= (1 << i);
+			}),
+		);
+
+		inited = true;
+		sync();
+
+		return function stop() {
+			run_all(unsubscribers);
+			cleanup();
+		};
+	});
+}
+
+export function get<T>(store: Readable<T>): T {
+	let value: T | undefined;
+	store.subscribe((_: T) => value = _)();
+	return value as T;
+}
diff --git a/store.mjs b/store.mjs
deleted file mode 100644
index 624ede3dde64..000000000000
--- a/store.mjs
+++ /dev/null
@@ -1,85 +0,0 @@
-import { run_all, noop, get_store_value, safe_not_equal } from './internal';
-
-export function readable(value, start) {
-	return {
-		subscribe: writable(value, start).subscribe
-	};
-}
-
-export function writable(value, start = noop) {
-	let stop;
-	const subscribers = [];
-
-	function set(new_value) {
-		if (safe_not_equal(value, new_value)) {
-			value = new_value;
-			if (!stop) return; // not ready
-			subscribers.forEach(s => s[1]());
-			subscribers.forEach(s => s[0](value));
-		}
-	}
-
-	function update(fn) {
-		set(fn(value));
-	}
-
-	function subscribe(run, invalidate = noop) {
-		const subscriber = [run, invalidate];
-		subscribers.push(subscriber);
-		if (subscribers.length === 1) stop = start(set) || noop;
-		run(value);
-
-		return () => {
-			const index = subscribers.indexOf(subscriber);
-			if (index !== -1) subscribers.splice(index, 1);
-			if (subscribers.length === 0) stop();
-		};
-	}
-
-	return { set, update, subscribe };
-}
-
-export function derived(stores, fn, initial_value) {
-	const single = !Array.isArray(stores);
-	if (single) stores = [stores];
-
-	const auto = fn.length < 2;
-	let value = {};
-
-	return readable(initial_value, set => {
-		let inited = false;
-		const values = [];
-
-		let pending = 0;
-		let cleanup = noop;
-
-		const sync = () => {
-			if (pending) return;
-			cleanup();
-			const result = fn(single ? values[0] : values, set);
-			if (auto) set(result);
-			else cleanup = result || noop;
-		};
-
-		const unsubscribers = stores.map((store, i) => store.subscribe(
-			value => {
-				values[i] = value;
-				pending &= ~(1 << i);
-				if (inited) sync();
-			},
-			() => {
-				pending |= (1 << i);
-			})
-		);
-
-		inited = true;
-		sync();
-
-		return function stop() {
-			run_all(unsubscribers);
-			cleanup();
-		};
-	});
-}
-
-export { get_store_value as get };
diff --git a/test/store/index.js b/test/store.ts
similarity index 89%
rename from test/store/index.js
rename to test/store.ts
index 5f9176cffb29..f931a841d741 100644
--- a/test/store/index.js
+++ b/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 +242,23 @@ 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}`);
+
+			assert.deepEqual(get(c), 'one 1');
+
+			a.set('two');
+			b.set(2);
+			assert.deepEqual(get(c), 'two 2');
+		});
 	});
 
 	describe('get', () => {
 		it('gets the current value of a store', () => {
-			const store = readable(42, () => {});
+			const store = readable(42, () => { });
 			assert.equal(get(store), 42);
 		});
 	});