chore: make store initialization logic simpler (#12281)

* chore: make store initialization logic simpler

* simpler still

* add comment

.changeset/fair-beers-help.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: make store initialization logic simpler

packages/svelte/src/internal/client/reactivity/sources.js

@@ -24,7 +24,6 @@ import {
 	UNOWNED,
 	MAYBE_DIRTY
 } from '../constants.js';
-import { UNINITIALIZED } from '../../../constants.js';
 import * as e from '../errors.js';
 
 let inspect_effects = new Set();
@@ -84,14 +83,7 @@ export function mutate(source, value) {
  * @returns {V}
  */
 export function set(source, value) {
-	var initialized = source.v !== UNINITIALIZED;
-
-	if (
-		initialized &&
-		current_reaction !== null &&
-		is_runes() &&
-		(current_reaction.f & DERIVED) !== 0
-	) {
+	if (current_reaction !== null && is_runes() && (current_reaction.f & DERIVED) !== 0) {
 		e.state_unsafe_mutation();
 	}
 
@@ -112,7 +104,6 @@ export function set(source, value) {
 		// We additionally want to skip this logic when initialising store sources
 		if (
 			is_runes() &&
-			initialized &&
 			current_effect !== null &&
 			(current_effect.f & CLEAN) !== 0 &&
 			(current_effect.f & BRANCH_EFFECT) === 0

packages/svelte/src/internal/client/reactivity/store.js

@@ -2,7 +2,6 @@
 /** @import { Store } from '#shared' */
 import { subscribe_to_store } from '../../../store/utils.js';
 import { noop } from '../../shared/utils.js';
-import { UNINITIALIZED } from '../../../constants.js';
 import { get } from '../runtime.js';
 import { teardown } from './effects.js';
 import { mutable_source, set } from './sources.js';
@@ -20,7 +19,7 @@ import { mutable_source, set } from './sources.js';
 export function store_get(store, store_name, stores) {
 	const entry = (stores[store_name] ??= {
 		store: null,
-		source: mutable_source(UNINITIALIZED),
+		source: mutable_source(undefined),
 		unsubscribe: noop
 	});
 
@@ -32,7 +31,18 @@ export function store_get(store, store_name, stores) {
 			set(entry.source, undefined);
 			entry.unsubscribe = noop;
 		} else {
-			entry.unsubscribe = subscribe_to_store(store, (v) => set(entry.source, v));
+			var initial = true;
+
+			entry.unsubscribe = subscribe_to_store(store, (v) => {
+				if (initial) {
+					// if the first time the store value is read is inside a derived,
+					// we will hit the `state_unsafe_mutation` error if we `set` the value
+					entry.source.v = v;
+					initial = false;
+				} else {
+					set(entry.source, v);
+				}
+			});
 		}
 	}