Add readonly to stores

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

@@ -411,6 +411,29 @@ import { get } from 'svelte/store';
 const value = get(store);
 ```
 
+#### `readonly`
+
+```js
+readableStore = readonly(writableStore);
+```
+
+---
+
+This simple helper function makes a store readonly. You can still subscribe to the changes from the original one using this new readable store.
+
+
+```js
+import { readonly } from 'svelte/store';
+
+const writableStore = writable(1);
+const readableStore = readonly(writableStore);
+
+readableStore.subscribe(console.log);
+
+writableStore.set(2); // console: 2
+readableStore.set(2); // ERROR
+```
+
 
 ### `svelte/motion`
 
@@ -788,7 +811,7 @@ The `crossfade` function creates a pair of [transitions](docs#transition_fn) cal
 * `delay` (`number`, default 0) — milliseconds before starting
 * `duration` (`number` | `function`, default 800) — milliseconds the transition lasts
 * `easing` (`function`, default `cubicOut`) — an [easing function](docs#svelte_easing)
-* `fallback` (`function`) — A fallback [transition](docs#transition_fn) to use for send when there is no matching element being received, and for receive when there is no element being sent. 
+* `fallback` (`function`) — A fallback [transition](docs#transition_fn) to use for send when there is no matching element being received, and for receive when there is no element being sent.
 
 ```sv
 <script>

src/runtime/store/index.ts

@@ -211,6 +211,17 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea
 	});
 }
 
+/**
+ * Takes a store and returns a new one derived from the old one that is readable.
+ *
+ * @param store - store to make readonly
+ */
+export function readonly<T>(store: Readable<T>): Readable<T> {
+	return {
+		subscribe: store.subscribe
+	};
+}
+
 /**
  * Get the current value from a store by subscribing and immediately unsubscribing.
  * @param store readable

test/store/index.ts

@@ -1,5 +1,5 @@
 import * as assert from 'assert';
-import { readable, writable, derived, get } from '../../store';
+import { readable, writable, derived, get, readonly } from '../../store';
 
 describe('store', () => {
 	describe('writable', () => {
@@ -411,4 +411,21 @@ describe('store', () => {
 			assert.equal(get(fake_observable), 42);
 		});
 	});
+
+	describe('readonly', () => {
+		it('makes a store readonly', () => {
+			const writableStore = writable(1);
+			const readableStore = readonly(writableStore);
+
+			assert.equal(get(readableStore), get(writableStore));
+
+			writableStore.set(2);
+
+			assert.equal(get(readableStore), 2);
+			assert.equal(get(readableStore), get(writableStore));
+
+			assert.throws(() => readableStore.set(3));
+		});
+
+	});
 });