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 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`]($effect) instead.
Here, the `$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 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.