add initial_value argument to derived store

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

@@ -293,13 +293,13 @@ const time = readable(new Date(), set => {
 store = derived(a, callback: (a: any) => any)
 ```
 ```js
-store = derived(a, callback: (a: any, set: (value: any) => void) => void)
+store = derived(a, callback: (a: any, set: (value: any) => void) => void, initial_value: any)
 ```
 ```js
 store = derived([a, ...b], callback: ([a: any, ...b: any[]]) => any)
 ```
 ```js
-store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void) => void)
+store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void) => void, initial_value: any)
 ```
 
 ---
@@ -318,12 +318,14 @@ const doubled = derived(a, $a => $a * 2);
 
 The callback can set a value asynchronously by accepting a second argument, `set`, and calling it when appropriate.
 
+In this case, you can also pass a third argument to `derived` — the initial value of the derived store before `set` is first called.
+
 ```js
 import { derived } from 'svelte/store';
 
 const delayed = derived(a, ($a, set) => {
 	setTimeout(() => set($a), 1000);
-});
+}, 'one moment...');
 ```
 
 ---

store.mjs

@@ -39,14 +39,14 @@ export function writable(value, start = noop) {
 	return { set, update, subscribe };
 }
 
-export function derived(stores, fn) {
+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(undefined, set => {
+	return readable(initial_value, set => {
 		let inited = false;
 		const values = [];
 
@@ -56,7 +56,7 @@ export function derived(stores, fn) {
 			if (pending) return;
 			const result = fn(single ? values[0] : values, set);
 			if (auto && (value !== (value = result))) set(result);
-		}
+		};
 
 		const unsubscribers = stores.map((store, i) => store.subscribe(
 			value => {

test/store/index.js

@@ -142,10 +142,10 @@ describe('store', () => {
 		});
 
 		it('passes optional set function', () => {
-			const number = writable(0);
+			const number = writable(1);
 			const evens = derived(number, (n, set) => {
 				if (n % 2 === 0) set(n);
-			});
+			}, 0);
 
 			const values = [];
 
@@ -153,17 +153,17 @@ describe('store', () => {
 				values.push(value);
 			});
 
-			number.set(1);
 			number.set(2);
 			number.set(3);
 			number.set(4);
+			number.set(5);
 			assert.deepEqual(values, [0, 2, 4]);
 
 			unsubscribe();
 
-			number.set(5);
 			number.set(6);
 			number.set(7);
+			number.set(8);
 			assert.deepEqual(values, [0, 2, 4]);
 		});