From 77434b2f1a8ece6eb084222c3e28b999507ab209 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 7 Jan 2025 17:26:06 +0100
Subject: [PATCH] docs: add docs on state_unsafe_mutation error

closes #14752
---
 .../98-reference/.generated/client-errors.md  | 24 +++++++++++++++++++
 .../svelte/messages/client-errors/errors.md   | 24 +++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/documentation/docs/98-reference/.generated/client-errors.md b/documentation/docs/98-reference/.generated/client-errors.md
index 2c2e0707ea12..0beb3cb9a96c 100644
--- a/documentation/docs/98-reference/.generated/client-errors.md
+++ b/documentation/docs/98-reference/.generated/client-errors.md
@@ -133,3 +133,27 @@ Reading state that was created inside the same derived is forbidden. Consider us
 ```
 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:
+
+```svelte
+<script>
+    let count = $state(0);
+    let multiple = $derived.by(() => {
+        const result = count * 2;
+        if (result > 10) {
+            count = 0;
+        }
+        return result;
+    });
+</script>
+
+<button onclick={() => count++}>{count} / {multiple}</button>
+```
+
+Here, the `$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.
+
+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
diff --git a/packages/svelte/messages/client-errors/errors.md b/packages/svelte/messages/client-errors/errors.md
index ce1f222c63ea..ab4d1519c18c 100644
--- a/packages/svelte/messages/client-errors/errors.md
+++ b/packages/svelte/messages/client-errors/errors.md
@@ -87,3 +87,27 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 ## state_unsafe_mutation
 
 > 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:
+
+```svelte
+<script>
+    let count = $state(0);
+    let multiple = $derived.by(() => {
+        const result = count * 2;
+        if (result > 10) {
+            count = 0;
+        }
+        return result;
+    });
+</script>
+
+<button onclick={() => count++}>{count} / {multiple}</button>
+```
+
+Here, the `$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.
+
+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