Sync svelte docs (#1280)

sync svelte docs

Co-authored-by: svelte-docs-bot[bot] <196124396+svelte-docs-bot[bot]@users.noreply.github.com>

Author: github-actions[bot]

apps/svelte.dev/content/docs/svelte/01-introduction/04-svelte-js-files.md

@@ -5,7 +5,7 @@ title: .svelte.js and .svelte.ts files
 
 Besides `.svelte` files, Svelte also operates on `.svelte.js` and `.svelte.ts` files.
 
-These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app.
+These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app (though note that you [cannot export reassigned state]($state#Passing-state-across-modules)).
 
 > [!LEGACY]
 > This is a concept that didn't exist prior to Svelte 5

apps/svelte.dev/content/docs/svelte/02-runes/02-$state.md

@@ -251,3 +251,83 @@ console.log(total.value); // 7
 ```
 
 ...though if you find yourself writing code like that, consider using [classes](#Classes) instead.
+
+## Passing state across modules
+
+You can declare state in `.svelte.js` and `.svelte.ts` files, but you can only _export_ that state if it's not directly reassigned. In other words you can't do this:
+
+```js
+/// file: state.svelte.js
+export let count = $state(0);
+
+export function increment() {
+	count += 1;
+}
+```
+
+That's because every reference to `count` is transformed by the Svelte compiler — the code above is roughly equivalent to this:
+
+```js
+/// file: state.svelte.js (compiler output)
+// @filename: index.ts
+interface Signal<T> {
+	value: T;
+}
+
+interface Svelte {
+	state<T>(value?: T): Signal<T>;
+	get<T>(source: Signal<T>): T;
+	set<T>(source: Signal<T>, value: T): void;
+}
+declare const $: Svelte;
+// ---cut---
+export let count = $.state(0);
+
+export function increment() {
+	$.set(count, $.get(count) + 1);
+}
+```
+
+> [!NOTE] You can see the code Svelte generates by clicking the 'JS Output' tab in the [playground](/playground).
+
+Since the compiler only operates on one file at a time, if another file imports `count` Svelte doesn't know that it needs to wrap each reference in `$.get` and `$.set`:
+
+```js
+// @filename: state.svelte.js
+export let count = 0;
+
+// @filename: index.js
+// ---cut---
+import { count } from './state.svelte.js';
+
+console.log(typeof count); // 'object', not 'number'
+```
+
+This leaves you with two options for sharing state between modules — either don't reassign it...
+
+```js
+// This is allowed — since we're updating
+// `counter.count` rather than `counter`,
+// Svelte doesn't wrap it in `$.state`
+export const counter = $state({
+	count: 0
+});
+
+export function increment() {
+	counter.count += 1;
+}
+```
+
+...or don't directly export it:
+
+```js
+let count = $state(0);
+
+export function getCount() {
+	return count;
+}
+
+export function increment() {
+	count += 1;
+}
+```

apps/svelte.dev/content/docs/svelte/02-runes/04-$effect.md

@@ -26,7 +26,7 @@ You can create an effect with the `$effect` rune ([demo](/playground/untitled#H4
 	});
 </script>
 
-<canvas bind:this={canvas} width="100" height="100" />
+<canvas bind:this={canvas} width="100" height="100"></canvas>
 ```
 
 When Svelte runs an effect function, it tracks which pieces of state (and derived state) are accessed (unless accessed inside [`untrack`](svelte#untrack)), and re-runs the function when that state later changes.
@@ -136,19 +136,33 @@ An effect only reruns when the object it reads changes, not when a property insi
 
 An effect only depends on the values that it read the last time it ran. This has interesting implications for effects that have conditional code.
 
-For instance, if `a` is `true` in the code snippet below, the code inside the `if` block will run and `b` will be evaluated. As such, changes to either `a` or `b` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE3VQzWrDMAx-FdUU4kBp71li6EPstOxge0ox8-QQK2PD-N1nLy2F0Z2Evj9_chKkP1B04pnYscc3cRCT8xhF95IEf8-Vq0DBr8rzPB_jJ3qumNERH-E2ECNxiRF9tIubWY00lgcYNAywj6wZJS8rtk83wjwgCrXHaULLUrYwKEgVGrnkx-Dx6MNFNstK5OjSbFGbwE0gdXuT_zGYrjmAuco515Hr1p_uXak3K3MgCGS9s-9D2grU-judlQYXIencnzad-tdR79qZrMyvw9wd5Z8Yv1h09dz8mn8AkM7Pfo0BAAA=).
+For instance, if `condition` is `true` in the code snippet below, the code inside the `if` block will run and `color` will be evaluated. As such, changes to either `condition` or `color` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE21RQW6DMBD8ytaNBJHaJFLViwNIVZ8RcnBgXVk1xsILTYT4e20TQg89IOPZ2fHM7siMaJBx9tmaWpFqjQNlAKXEihx7YVJpdIyfRkY3G4gB8Pi97cPanRtQU8AuwuF_eNUaQuPlOMtc1SlLRWlKUo1tOwJflUikQHZtA0klzCDc64Imx0ANn8bInV1CDhtHgjClrsftcSXotluLybOUb3g4JJHhOZs5WZpuIS9gjNqkJKQP5e2ClrR4SMdZ13E4xZ8zTPOTJU2A2uE_PQ9COCI926_hTVarIU4hu_REPlBrKq2q73ycrf1N-vS4TMUsulaVg3EtR8H9rFgsg8uUsT1B2F9eshigZHBRpuaD0D3mY8Qm2BfB5N2YyRzdNEYVDy0Ja-WsFjcOUuP1HvFLWA6H3XuHTUSmmDV2--0TXonxsKbp7G9C6R__NONS-MFNvxj_d6mBAgAA).
 
-Conversely, if `a` is `false`, `b` will not be evaluated, and the effect will _only_ re-run when `a` changes.
+Conversely, if `condition` is `false`, `color` will not be evaluated, and the effect will _only_ re-run again when `condition` changes.
 
 ```ts
-let a = false;
-let b = false;
+// @filename: ambient.d.ts
+declare module 'canvas-confetti' {
+	interface ConfettiOptions {
+		colors: string[];
+	}
+
+	function confetti(opts?: ConfettiOptions): void;
+	export default confetti;
+}
+
+// @filename: index.js
 // ---cut---
-$effect(() => {
-	console.log('running');
+import confetti from 'canvas-confetti';
 
-	if (a) {
-		console.log('b:', b);
+let condition = $state(true);
+let color = $state('#ff3e00');
+
+$effect(() => {
+	if (condition) {
+		confetti({ colors: [color] });
+	} else {
+		confetti();
 	}
 });
 ```
@@ -212,20 +226,19 @@ It is used to implement abstractions like [`createSubscriber`](/docs/svelte/svel
 
 The `$effect.root` rune is an advanced feature that creates a non-tracked scope that doesn't auto-cleanup. This is useful for nested effects that you want to manually control. This rune also allows for the creation of effects outside of the component initialisation phase.
 
-```svelte
-<script>
-	let count = $state(0);
+```js
+const destroy = $effect.root(() => {
+	$effect(() => {
+		// setup
+	});
 
-	const cleanup = $effect.root(() => {
-		$effect(() => {
-			console.log(count);
-		});
+	return () => {
+		// cleanup
+	};
+});
 
-		return () => {
-			console.log('effect root cleanup');
-		};
-	});
-</script>
+// later...
+destroy();
 ```
 
 ## When not to use `$effect`

apps/svelte.dev/content/docs/svelte/03-template-syntax/13-transition.md

@@ -23,10 +23,6 @@ The `transition:` directive indicates a _bidirectional_ transition, which means
 {/if}
 ```
 
-## Built-in transitions
-
-A selection of built-in transitions can be imported from the [`svelte/transition`](svelte-transition) module.
-
 ## Local vs global
 
 Transitions are local by default. Local transitions only play when the block they belong to is created or destroyed, _not_ when parent blocks are created or destroyed.
@@ -41,6 +37,10 @@ Transitions are local by default. Local transitions only play when the block the
 {/if}
 ```
 
+## Built-in transitions
+
+A selection of built-in transitions can be imported from the [`svelte/transition`](svelte-transition) module.
+
 ## Transition parameters
 
 Transitions can have parameters.