fix: allow writable/readable empty initialization

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Svelte changelog
 
+## Unreleased
+
+* Fix type signatures of `writable` and `readable`. It's possible to invoke them without arguments ([#6291](https://github.com/sveltejs/svelte/issues/6291), [#6345](https://github.com/sveltejs/svelte/issues/6345))
+
 ## 3.38.2
 
 * Revert hydration optimisation for the time being ([#6279](https://github.com/sveltejs/svelte/issues/6279))

site/content/docs/03-run-time.md

@@ -245,10 +245,10 @@ This makes it possible to wrap almost any other reactive state handling library
 #### `writable`
 
 ```js
-store = writable(value: any)
+store = writable(value?: any)
 ```
 ```js
-store = writable(value: any, (set: (value: any) => void) => () => void)
+store = writable(value?: any, start?: (set: (value: any) => void) => () => void)
 ```
 
 ---
@@ -297,14 +297,12 @@ unsubscribe(); // logs 'no more subscribers'
 #### `readable`
 
 ```js
-store = readable(value: any, (set: (value: any) => void) => () => void)
+store = readable(value?: any, start?: (set: (value: any) => void) => () => void)
 ```
 
 ---
 
-Creates a store whose value cannot be set from 'outside', the first argument is the store's initial value.
-
-The second argument to `readable` is the same as the second argument to `writable`, except that it is required with `readable` (since otherwise there would be no way to update the store value).
+Creates a store whose value cannot be set from 'outside', the first argument is the store's initial value, and the second argument to `readable` is the same as the second argument to `writable`.
 
 ```js
 import { readable } from 'svelte/store';

src/runtime/store/index.ts

@@ -50,7 +50,7 @@ const subscriber_queue = [];
  * @param value initial value
  * @param {StartStopNotifier}start start and stop notifications for subscriptions
  */
-export function readable<T>(value: T, start: StartStopNotifier<T>): Readable<T> {
+export function readable<T>(value?: T, start?: StartStopNotifier<T>): Readable<T> {
 	return {
 		subscribe: writable(value, start).subscribe
 	};
@@ -61,7 +61,7 @@ export function readable<T>(value: T, start: StartStopNotifier<T>): Readable<T>
  * @param {*=}value initial value
  * @param {StartStopNotifier=}start start and stop notifications for subscriptions
  */
-export function writable<T>(value: T, start: StartStopNotifier<T> = noop): Writable<T> {
+export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writable<T> {
 	let stop: Unsubscriber;
 	const subscribers: Array<SubscribeInvalidateTuple<T>> = [];
 

test/store/index.js → test/store/index.ts

@@ -22,6 +22,19 @@ describe('store', () => {
 			assert.deepEqual(values, [0, 1, 2]);
 		});
 
+		it('creates an undefined writable store', () => {
+			const store = writable();
+			const values = [];
+
+			const unsubscribe = store.subscribe(value => {
+				values.push(value);
+			});
+
+			unsubscribe();
+
+			assert.deepEqual(values, [undefined]);
+		});
+
 		it('calls provided subscribe handler', () => {
 			let called = 0;
 
@@ -114,6 +127,32 @@ describe('store', () => {
 
 			assert.deepEqual(values, [0, 1, 2]);
 		});
+
+		it('creates an undefined readable store', () => {
+			const store = readable();
+			const values = [];
+
+			const unsubscribe = store.subscribe(value => {
+				values.push(value);
+			});
+
+			unsubscribe();
+
+			assert.deepEqual(values, [undefined]);
+		});
+
+		it('creates a readable store without updater', () => {
+			const store = readable(100);
+			const values = [];
+
+			const unsubscribe = store.subscribe(value => {
+				values.push(value);
+			});
+
+			unsubscribe();
+
+			assert.deepEqual(values, [100]);
+		});
 	});
 
 	const fake_observable = {