From 7881764aa5fa9253ee33a698e772a2f4483c13d9 Mon Sep 17 00:00:00 2001
From: "svelte-docs-bot[bot]"
 <196124396+svelte-docs-bot[bot]@users.noreply.github.com>
Date: Fri, 21 Mar 2025 17:07:11 +0000
Subject: [PATCH] sync svelte docs

---
 .../docs/svelte/02-runes/03-$derived.md       | 42 +++++++++++++++++++
 .../docs/svelte/02-runes/04-$effect.md        |  2 +
 .../98-reference/.generated/client-errors.md  | 37 +++++++++-------
 .../svelte/98-reference/30-runtime-errors.md  | 37 +++++++++-------
 4 files changed, 88 insertions(+), 30 deletions(-)

diff --git a/apps/svelte.dev/content/docs/svelte/02-runes/03-$derived.md b/apps/svelte.dev/content/docs/svelte/02-runes/03-$derived.md
index 1f63b3fe95..c482c8d9cb 100644
--- a/apps/svelte.dev/content/docs/svelte/02-runes/03-$derived.md
+++ b/apps/svelte.dev/content/docs/svelte/02-runes/03-$derived.md
@@ -53,6 +53,48 @@ Anything read synchronously inside the `$derived` expression (or `$derived.by` f
 
 To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).
 
+## Overriding derived values
+
+Derived expressions are recalculated when their dependencies change, but you can temporarily override their values by reassigning them (unless they are declared with `const`). This can be useful for things like _optimistic UI_, where a value is derived from the 'source of truth' (such as data from your server) but you'd like to show immediate feedback to the user:
+
+```svelte
+<script>
+	let { post, like } = $props();
+
+	let likes = $derived(post.likes);
+
+	async function onclick() {
+		// increment the `likes` count immediately...
+		likes += 1;
+
+		// and tell the server, which will eventually update `post`
+		try {
+			await like();
+		} catch {
+			// failed! roll back the change
+			likes -= 1;
+		}
+	}
+</script>
+
+<button {onclick}>🧡 {likes}</button>
+```
+
+> [!NOTE] Prior to Svelte 5.25, deriveds were read-only.
+
+## Deriveds and reactivity
+
+Unlike `$state`, which converts objects and arrays to [deeply reactive proxies]($state#Deep-state), `$derived` values are left as-is. For example, [in a case like this](/playground/untitled#H4sIAAAAAAAAE4VU22rjMBD9lUHd3aaQi9PdstS1A3t5XvpQ2Ic4D7I1iUUV2UjjNMX431eS7TRdSosxgjMzZ45mjt0yzffIYibvy0ojFJWqDKCQVBk2ZVup0LJ43TJ6rn2aBxw-FP2o67k9oCKP5dziW3hRaUJNjoYltjCyplWmM1JIIAn3FlL4ZIkTTtYez6jtj4w8WwyXv9GiIXiQxLVs9pfTMR7EuoSLIuLFbX7Z4930bZo_nBrD1bs834tlfvsBz9_SyX6PZXu9XaL4gOWn4sXjeyzftv4ZWfyxubpzxzg6LfD4MrooxELEosKCUPigQCMPKCZh0OtQE1iSxcsmdHuBvCiHZXALLXiN08EL3RRkaJ_kDVGle0HcSD5TPEeVtj67O4Nrg9aiSNtBY5oODJkrL5QsHtN2cgXp6nSJMWzpWWGasdlsGEMbzi5jPr5KFr0Ep7pdeM2-TCelCddIhDxAobi1jqF3cMaC1RKp64bAW9iFAmXGIHfd4wNXDabtOLN53w8W53VvJoZLh7xk4Rr3CoL-UNoLhWHrT1JQGcM17u96oES5K-kc2XOzkzqGCKL5De79OUTyyrg1zgwXsrEx3ESfx4Bz0M5UjVMHB24mw9SuXtXFoN13fYKOM1tyUT3FbvbWmSWCZX2Er-41u5xPoml45svRahl9Wb9aasbINJixDZwcPTbyTLZSUsAvrg_cPuCR7s782_WU8343Y72Qtlb8OYatwuOQvuN13M_hJKNfxann1v1U_B1KZ_D_mzhzhz24fw85CSz2irtN9w9HshBK7AQAAA==)...
+
+```svelte
+let items = $state([...]);
+
+let index = $state(0);
+let selected = $derived(items[index]);
+```
+
+...you can change (or `bind:` to) properties of `selected` and it will affect the underlying `items` array. If `items` was _not_ deeply reactive, mutating `selected` would have no effect.
+
 ## 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').
diff --git a/apps/svelte.dev/content/docs/svelte/02-runes/04-$effect.md b/apps/svelte.dev/content/docs/svelte/02-runes/04-$effect.md
index d9d45a2075..decb34866a 100644
--- a/apps/svelte.dev/content/docs/svelte/02-runes/04-$effect.md
+++ b/apps/svelte.dev/content/docs/svelte/02-runes/04-$effect.md
@@ -255,6 +255,8 @@ In general, `$effect` is best considered something of an escape hatch — useful
 
 > [!NOTE] For things that are more complicated than a simple expression like `count * 2`, you can also use `$derived.by`.
 
+If you're using an effect because you want to be able to reassign the derived value (to build an optimistic UI, for example) note that [deriveds can be directly overridden]($derived#Overriding-derived-values) as of Svelte 5.25.
+
 You might be tempted to do something convoluted with effects to link one value to another. The following example shows two inputs for "money spent" and "money left" that are connected to each other. If you update one, the other should update accordingly. Don't use effects for this ([demo](/playground/untitled#H4sIAAAAAAAACpVRy26DMBD8FcvKgUhtoIdeHBwp31F6MGSJkBbHwksEQvx77aWQqooq9bgzOzP7mGTdIHipPiZJowOpGJAv0po2VmfnDv4OSBErjYdneHWzBJaCjcx91TWOToUtCIEE3cig0OIty44r5l1oDtjOkyFIsv3GINQ_CNYyGegd1DVUlCR7oU9iilDUcP8S8roYs9n8p2wdYNVFm4csTx872BxNCcjr5I11fdgonEkXsjP2CoUUZWMv6m6wBz2x7yxaM-iJvWeRsvSbSVeUy5i0uf8vKA78NIeJLSZWv1I8jQjLdyK4XuTSeIdmVKJGGI4LdjVOiezwDu1yG74My8PLCQaSiroe5s_5C2PHrkVGAgAA)):
 
 ```svelte
diff --git a/apps/svelte.dev/content/docs/svelte/98-reference/.generated/client-errors.md b/apps/svelte.dev/content/docs/svelte/98-reference/.generated/client-errors.md
index 62d9c3302a..fd9419176d 100644
--- a/apps/svelte.dev/content/docs/svelte/98-reference/.generated/client-errors.md
+++ b/apps/svelte.dev/content/docs/svelte/98-reference/.generated/client-errors.md
@@ -128,26 +128,33 @@ Cannot set prototype of `$state` object
 Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
 ```
 
-This error is thrown in a situation like this:
+This error occurs when state is updated while evaluating a `$derived`. You might encounter it while trying to 'derive' two pieces of state in one go:
 
 ```svelte
 <script>
-    let count = $state(0);
-    let multiple = $derived.by(() => {
-        const result = count * 2;
-        if (result > 10) {
-            count = 0;
-        }
-        return result;
-    });
+	let count = $state(0);
+
+	let even = $state(true);
+
+	let odd = $derived.by(() => {
+		even = count % 2 === 0;
+		return !even;
+	});
 </script>
 
-<button onclick={() => count++}>{count} / {multiple}</button>
+<button onclick={() => count++}>{count}</button>
+
+<p>{count} is even: {even}</p>
+<p>{count} is odd: {odd}</p>
 ```
 
-Here, the `$derived` updates `count`, which is `$state` and therefore forbidden to do. It is forbidden because the reactive graph could become unstable as a result, leading to subtle bugs, like values being stale or effects firing in the wrong order. To prevent this, Svelte errors when detecting an update to a `$state` variable.
+This is forbidden because it introduces instability: if `<p>{count} is even: {even}</p>` is updated before `odd` is recalculated, `even` will be stale. In most cases the solution is to make everything derived:
+
+```js
+let count = 0;
+// ---cut---
+let even = $derived(count % 2 === 0);
+let odd = $derived(!even);
+```
 
-To fix this:
-- See if it's possible to refactor your `$derived` such that the update becomes unnecessary
-- Think about why you need to update `$state` inside a `$derived` in the first place. Maybe it's because you're using `bind:`, which leads you down a bad code path, and separating input and output path (by splitting it up to an attribute and an event, or by using [Function bindings](bind#Function-bindings)) makes it possible avoid the update
-- If it's unavoidable, you may need to use an [`$effect`]($effect) instead. This could include splitting parts of the `$derived` into an [`$effect`]($effect) which does the updates
+If side-effects are unavoidable, use [`$effect`]($effect) instead.
diff --git a/apps/svelte.dev/content/docs/svelte/98-reference/30-runtime-errors.md b/apps/svelte.dev/content/docs/svelte/98-reference/30-runtime-errors.md
index a2eef7743b..ef6627c1c7 100644
--- a/apps/svelte.dev/content/docs/svelte/98-reference/30-runtime-errors.md
+++ b/apps/svelte.dev/content/docs/svelte/98-reference/30-runtime-errors.md
@@ -135,29 +135,36 @@ Cannot set prototype of `$state` object
 Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
 ```
 
-This error is thrown in a situation like this:
+This error occurs when state is updated while evaluating a `$derived`. You might encounter it while trying to 'derive' two pieces of state in one go:
 
 ```svelte
 <script>
-    let count = $state(0);
-    let multiple = $derived.by(() => {
-        const result = count * 2;
-        if (result > 10) {
-            count = 0;
-        }
-        return result;
-    });
+	let count = $state(0);
+
+	let even = $state(true);
+
+	let odd = $derived.by(() => {
+		even = count % 2 === 0;
+		return !even;
+	});
 </script>
 
-<button onclick={() => count++}>{count} / {multiple}</button>
+<button onclick={() => count++}>{count}</button>
+
+<p>{count} is even: {even}</p>
+<p>{count} is odd: {odd}</p>
 ```
 
-Here, the `$derived` updates `count`, which is `$state` and therefore forbidden to do. It is forbidden because the reactive graph could become unstable as a result, leading to subtle bugs, like values being stale or effects firing in the wrong order. To prevent this, Svelte errors when detecting an update to a `$state` variable.
+This is forbidden because it introduces instability: if `<p>{count} is even: {even}</p>` is updated before `odd` is recalculated, `even` will be stale. In most cases the solution is to make everything derived:
+
+```js
+let count = 0;
+// ---cut---
+let even = $derived(count % 2 === 0);
+let odd = $derived(!even);
+```
 
-To fix this:
-- See if it's possible to refactor your `$derived` such that the update becomes unnecessary
-- Think about why you need to update `$state` inside a `$derived` in the first place. Maybe it's because you're using `bind:`, which leads you down a bad code path, and separating input and output path (by splitting it up to an attribute and an event, or by using [Function bindings](bind#Function-bindings)) makes it possible avoid the update
-- If it's unavoidable, you may need to use an [`$effect`]($effect) instead. This could include splitting parts of the `$derived` into an [`$effect`]($effect) which does the updates
+If side-effects are unavoidable, use [`$effect`]($effect) instead.
 
 
 ## Server errors