docs: add docs on state_unsafe_mutation error (#14932)

closes #14752
pull/15516/head
Simon H 6 months ago committed by GitHub
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@ -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

@ -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

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