print useful errors when subscribing to stores during SSR

.changeset/large-toes-fetch.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: print useful error when subscribing to SvelteKit's stores at the wrong time during SSR

packages/kit/src/runtime/app/stores.js

@@ -23,23 +23,23 @@ export const getStores = () => {
 export const page = {
 	/** @param {(value: any) => void} fn */
 	subscribe(fn) {
-		const store = getStores().page;
+		const store = __SVELTEKIT_DEV__ ? get_store('page') : getStores().page;
 		return store.subscribe(fn);
 	}
 };
 
 /** @type {typeof import('$app/stores').navigating} */
 export const navigating = {
 	subscribe(fn) {
-		const store = getStores().navigating;
+		const store = __SVELTEKIT_DEV__ ? get_store('navigating') : getStores().navigating;
 		return store.subscribe(fn);
 	}
 };
 
 /** @type {typeof import('$app/stores').updated} */
 export const updated = {
 	subscribe(fn) {
-		const store = getStores().updated;
+		const store = __SVELTEKIT_DEV__ ? get_store('updated') : getStores().updated;
 
 		if (browser) {
 			updated.check = store.check;
@@ -55,3 +55,18 @@ export const updated = {
 		);
 	}
 };
+
+/**
+ * @template {keyof ReturnType<typeof getStores>} Name
+ * @param {Name} name
+ * @returns {ReturnType<typeof getStores>[Name]}
+ */
+function get_store(name) {
+	try {
+		return getStores()[name];
+	} catch (e) {
+		throw new Error(
+			`Cannot subscribe to '${name}' store on the server outside of a Svelte component, as it is bound to the current request via component context. This prevents state from leaking between users.`
+		);
+	}
+}

packages/kit/types/ambient.d.ts

@@ -292,16 +292,22 @@ declare module '$app/stores' {
 
 	/**
 	 * A readable store whose value contains page data.
+	 *
+	 * On the server, this store can only be subscribed to during component initialization. In the browser, it can be subscribed to at any time.
 	 */
 	export const page: Readable<Page>;
 	/**
 	 * A readable store.
 	 * When navigating starts, its value is a `Navigation` object with `from`, `to`, `type` and (if `type === 'popstate'`) `delta` properties.
 	 * When navigating finishes, its value reverts to `null`.
+	 *
+	 * On the server, this store can only be subscribed to during component initialization. In the browser, it can be subscribed to at any time.
 	 */
 	export const navigating: Readable<Navigation | null>;
 	/**
-	 *  A readable store whose initial value is `false`. If [`version.pollInterval`](https://kit.svelte.dev/docs/configuration#version) is a non-zero value, SvelteKit will poll for new versions of the app and update the store value to `true` when it detects one. `updated.check()` will force an immediate check, regardless of polling.
+	 * A readable store whose initial value is `false`. If [`version.pollInterval`](https://kit.svelte.dev/docs/configuration#version) is a non-zero value, SvelteKit will poll for new versions of the app and update the store value to `true` when it detects one. `updated.check()` will force an immediate check, regardless of polling.
+	 *
+	 * On the server, this store can only be subscribed to during component initialization. In the browser, it can be subscribed to at any time.
 	 */
 	export const updated: Readable<boolean> & { check(): Promise<boolean> };