Sync svelte docs (#1095)

sync svelte docs

Co-authored-by: Rich-Harris <[email protected]>

apps/svelte.dev/content/docs/svelte/02-runes/03-$derived.md

@@ -52,3 +52,20 @@ In essence, `$derived(expression)` is equivalent to `$derived.by(() => expressio
 Anything read synchronously inside the `$derived` expression (or `$derived.by` function body) is considered a _dependency_ of the derived state. When the state changes, the derived will be marked as _dirty_ and recalculated when it is next read.
 
 To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).
+
+## Update propagation
+
+Svelte uses something called _push-pull reactivity_ — when state is updated, everything that depends on the state (whether directly or indirectly) is immediately notified of the change (the 'push'), but derived values are not re-evaluated until they are actually read (the 'pull').
+
+If the new value of a derived is referentially identical to its previous value, downstream updates will be skipped. In other words, Svelte will only update the text inside the button when `large` changes, not when `count` changes, even though `large` depends on `count`:
+
+```svelte
+<script>
+	let count = $state(0);
+	let large = $derived(count > 10);
+</script>
+
+<button onclick={() => count++}>
+	{large}
+</button>
+```

apps/svelte.dev/content/docs/svelte/07-misc/02-testing.md

@@ -41,7 +41,7 @@ You can now write unit tests for code inside your `.js/.ts` files:
 /// file: multiplier.svelte.test.js
 import { flushSync } from 'svelte';
 import { expect, test } from 'vitest';
-import { multiplier } from './multiplier.js';
+import { multiplier } from './multiplier.svelte.js';
 
 test('Multiplier', () => {
 	let double = multiplier(0, 2);
@@ -54,9 +54,30 @@ test('Multiplier', () => {
 });
 ```
 
+```js
+/// file: multiplier.svelte.js
+/**
+ * @param {number} initial
+ * @param {number} k
+ */
+export function multiplier(initial, k) {
+	let count = $state(initial);
+
+	return {
+		get value() {
+			return count * k;
+		},
+		/** @param {number} c */
+		set: (c) => {
+			count = c;
+		}
+	};
+}
+```
+
 ### Using runes inside your test files
 
-It is possible to use runes inside your test files. First ensure your bundler knows to route the file through the Svelte compiler before running the test by adding `.svelte` to the filename (e.g `multiplier.svelte.test.js`). After that, you can use runes inside your tests.
+Since Vitest processes your test files the same way as your source files, you can use runes inside your tests as long as the filename includes `.svelte`:
 
 ```js
 /// file: multiplier.svelte.test.js
@@ -76,6 +97,21 @@ test('Multiplier', () => {
 });
 ```
 
+```js
+/// file: multiplier.svelte.js
+/**
+ * @param {() => number} getCount
+ * @param {number} k
+ */
+export function multiplier(getCount, k) {
+	return {
+		get value() {
+			return getCount() * k;
+		}
+	};
+}
+```
+
 If the code being tested uses effects, you need to wrap the test inside `$effect.root`:
 
 ```js
@@ -106,6 +142,27 @@ test('Effect', () => {
 });
 ```
 
+```js
+/// file: logger.svelte.js
+/**
+ * @param {() => any} getValue
+ */
+export function logger(getValue) {
+	/** @type {any[]} */
+	let log = $state([]);
+
+	$effect(() => {
+		log.push(getValue());
+	});
+
+	return {
+		get value() {
+			return log;
+		}
+	};
+}
+```
+
 ### Component testing
 
 It is possible to test your components in isolation using Vitest.

apps/svelte.dev/content/docs/svelte/07-misc/04-custom-elements.md

@@ -126,3 +126,4 @@ Custom elements can be a useful way to package components for consumption in a n
 - The deprecated `let:` directive has no effect, because custom elements do not have a way to pass data to the parent component that fills the slot
 - Polyfills are required to support older browsers
 - You can use Svelte's context feature between regular Svelte components within a custom element, but you can't use them across custom elements. In other words, you can't use `setContext` on a parent custom element and read that with `getContext` in a child custom element.
+- Don't declare properties or attributes starting with `on`, as their usage will be interpreted as an event listener. In other words, Svelte treats `<custom-element oneworld={true}></custom-element>` as `customElement.addEventListener('eworld', true)` (and not as `customElement.oneworld = true`)