From b295d68ec696a00e4d52cc7f86fed149b13062d2 Mon Sep 17 00:00:00 2001
From: Ignatius Bagus <ignatius.mbs@gmail.com>
Date: Thu, 20 May 2021 20:31:51 +0700
Subject: [PATCH] fix types: allow writable/readable empty initialization
 (#6293)

Fixes #6291
Fixes #6345

Both writable and readable initialized without any arguments are already valid, but TS complains about it. This makes both allowed to be emptily initialized. It's also possible to invoke readable with one argument only.
---
 CHANGELOG.md                      |  4 ++++
 site/content/docs/03-run-time.md  | 10 ++++----
 src/runtime/store/index.ts        |  4 ++--
 test/store/{index.js => index.ts} | 39 +++++++++++++++++++++++++++++++
 4 files changed, 49 insertions(+), 8 deletions(-)
 rename test/store/{index.js => index.ts} (90%)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 141d5661bc39..89249634bbd2 100644
--- a/CHANGELOG.md
+++ b/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))
diff --git a/site/content/docs/03-run-time.md b/site/content/docs/03-run-time.md
index 7fbc8850de86..c0bd14531610 100644
--- a/site/content/docs/03-run-time.md
+++ b/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';
diff --git a/src/runtime/store/index.ts b/src/runtime/store/index.ts
index cf15db5250b9..213a4ff6fdc6 100644
--- a/src/runtime/store/index.ts
+++ b/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>> = [];
 
diff --git a/test/store/index.js b/test/store/index.ts
similarity index 90%
rename from test/store/index.js
rename to test/store/index.ts
index 2af4a6f35d85..b6fc5940e111 100644
--- a/test/store/index.js
+++ b/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 = {