Add `unbox` to stores

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

@@ -411,6 +411,42 @@ import { get } from 'svelte/store';
 const value = get(store);
 ```
 
+#### `unbox`
+
+```js
+unbox(store, object, "property")
+```
+
+---
+
+Unbox is a simple shorthand for assigning a store value to a property of an object. Shorthand for `store.subscribe(value => object.storeProp = value)`
+
+```js
+import { unbox } from 'svelte/store';
+
+const object = {
+	storeProp: null
+};
+
+unbox(store, object, "storeProp");
+```
+
+---
+
+It internally subscribes to the store, so you can also stop the unboxing:
+
+```js
+import { unbox } from 'svelte/store';
+
+const object = {
+	storeProp: null
+};
+
+const stop = unbox(store, object, "storeProp");
+
+stop();
+```
+
 
 ### `svelte/motion`
 
@@ -788,7 +824,7 @@ 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
 	});
 }
 
+/**
+ * "Unboxes" a store value to a property of an object.
+ *
+ * @param store - input store
+ * @param object - object with specified property
+ * @param property - a property of `object` to be assigned to
+ */
+export function unbox<T, K extends PropertyKey>(store: Readable<T>, object: { [X in K]: T }, property: K): Unsubscriber {
+	return store.subscribe(value => object[property] = value);
+}
+
 /**
  * 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, unbox } from '../../store';
 
 describe('store', () => {
 	describe('writable', () => {
@@ -411,4 +411,37 @@ describe('store', () => {
 			assert.equal(get(fake_observable), 42);
 		});
 	});
+
+	describe('unbox', () => {
+		it('unboxes a value', () => {
+			const store = writable(42);
+			const object = {
+				prop: 0
+			};
+
+			unbox(store, object, 'prop');
+			assert.equal(object.prop, 42);
+
+			store.set(123);
+			assert.equal(object.prop, 123);
+		});
+
+		it('can stop unboxing', () => {
+			const store = writable(42);
+			const object = {
+				prop: 0
+			};
+
+			const stop = unbox(store, object, 'prop');
+			assert.equal(object.prop, 42);
+
+			store.set(123);
+			assert.equal(object.prop, 123);
+
+			stop();
+
+			store.set(0);
+			assert.equal(object.prop, 123);
+		});
+	});
 });