## Client errors

<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->

### async_derived_orphan

```
Cannot create a `$derived(...)` with an `await` expression outside of an effect tree
```

In Svelte there are two types of reaction — [`$derived`](/docs/svelte/$derived) and [`$effect`](/docs/svelte/$effect). Deriveds can be created anywhere, because they run _lazily_ and can be [garbage collected](https://developer.mozilla.org/en-US/docs/Glossary/Garbage_collection) if nothing references them. Effects, by contrast, keep running eagerly whenever their dependencies change, until they are destroyed.

Because of this, effects can only be created inside other effects (or [effect roots](/docs/svelte/$effect#$effect.root), such as the one that is created when you first mount a component) so that Svelte knows when to destroy them.

Some sleight of hand occurs when a derived contains an `await` expression: Since waiting until we read `{await getPromise()}` to call `getPromise` would be too late, we use an effect to instead call it proactively, notifying Svelte when the value is available. But since we're using an effect, we can only create asynchronous deriveds inside another effect.

### bind_invalid_checkbox_value

```
Using `bind:value` together with a checkbox input is not allowed. Use `bind:checked` instead
```

### bind_invalid_export

```
Component %component% has an export named `%key%` that a consumer component is trying to access using `bind:%key%`, which is disallowed. Instead, use `bind:this` (e.g. `<%name% bind:this={component} />`) and then access the property on the bound component instance (e.g. `component.%key%`)
```

### bind_not_bindable

```
A component is attempting to bind to a non-bindable property `%key%` belonging to %component% (i.e. `<%name% bind:%key%={...}>`). To mark a property as bindable: `let { %key% = $bindable() } = $props()`
```

### component_api_changed

```
Calling `%method%` on a component instance (of %component%) is no longer valid in Svelte 5
```

See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information.

### component_api_invalid_new

```
Attempted to instantiate %component% with `new %name%`, which is no longer valid in Svelte 5. If this component is not under your control, set the `compatibility.componentApi` compiler option to `4` to keep it working.
```

See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information.

### derived_references_self

```
A derived value cannot reference itself recursively
```

### each_key_duplicate

```
Keyed each block has duplicate key at indexes %a% and %b%
```

```
Keyed each block has duplicate key `%value%` at indexes %a% and %b%
```

### effect_in_teardown

```
`%rune%` cannot be used inside an effect cleanup function
```

### effect_in_unowned_derived

```
Effect cannot be created inside a `$derived` value that was not itself created inside an effect
```

### effect_orphan

```
`%rune%` can only be used inside an effect (e.g. during component initialisation)
```

### effect_pending_outside_reaction

```
`$effect.pending()` can only be called inside an effect or derived
```

### effect_update_depth_exceeded

```
Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
```

### flush_sync_in_effect

```
Cannot use `flushSync` inside an effect
```

The `flushSync()` function can be used to flush any pending effects synchronously. It cannot be used if effects are currently being flushed — in other words, you can call it after a state change but _not_ inside an effect.

This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.

### get_abort_signal_outside_reaction

```
`getAbortSignal()` can only be called inside an effect or derived
```

### hydration_failed

```
Failed to hydrate the application
```

### invalid_snippet

```
Could not `{@render}` snippet due to the expression being `null` or `undefined`. Consider using optional chaining `{@render snippet?.()}`
```

### lifecycle_legacy_only

```
`%name%(...)` cannot be used in runes mode
```

### props_invalid_value

```
Cannot do `bind:%key%={undefined}` when `%key%` has a fallback value
```

### props_rest_readonly

```
Rest element properties of `$props()` such as `%property%` are readonly
```

### rune_outside_svelte

```
The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
```

### set_context_after_init

```
`setContext` must be called when a component first initializes, not in a subsequent effect or after an `await` expression
```

This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.

### state_descriptors_fixed

```
Property descriptors defined on `$state` objects must contain `value` and always be `enumerable`, `configurable` and `writable`.
```

### state_prototype_fixed

```
Cannot set prototype of `$state` object
```

### state_unsafe_mutation

```
Updating state inside `$derived(...)`, `$inspect(...)` or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
```

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 even = $state(true);

	let odd = $derived.by(() => {
		even = count % 2 === 0;
		return !even;
	});
</script>

<button onclick={() => count++}>{count}</button>

<p>{count} is even: {even}</p>
<p>{count} is odd: {odd}</p>
```

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);
```

If side-effects are unavoidable, use [`$effect`]($effect) instead.


## Server errors

<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->

### lifecycle_function_unavailable

```
`%name%(...)` is not available on the server
```

Certain methods such as `mount` cannot be invoked while running in a server context. Avoid calling them eagerly, i.e. not during render.


## Shared errors

<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->

### await_outside_boundary

```
Cannot await outside a `<svelte:boundary>` with a `pending` snippet
```

The `await` keyword can only appear in a `$derived(...)` or template expression, or at the top level of a component's `<script>` block, if it is inside a [`<svelte:boundary>`](/docs/svelte/svelte-boundary) that has a `pending` snippet:

```svelte
<svelte:boundary>
	<p>{await getData()}</p>

	{#snippet pending()}
		<p>loading...</p>
	{/snippet}
</svelte:boundary>
```

This restriction may be lifted in a future version of Svelte.

### invalid_default_snippet

```
Cannot use `{@render children(...)}` if the parent component uses `let:` directives. Consider using a named snippet instead
```

This error would be thrown in a setup like this:

```svelte
<!--- file: Parent.svelte --->
<List {items} let:entry>
    <span>{entry}</span>
</List>
```

```svelte
<!--- file: List.svelte --->
<script>
    let { items, children } = $props();
</script>

<ul>
    {#each items as item}
        <li>{@render children(item)}</li>
    {/each}
</ul>
```

Here, `List.svelte` is using `{@render children(item)` which means it expects `Parent.svelte` to use snippets. Instead, `Parent.svelte` uses the deprecated `let:` directive. This combination of APIs is incompatible, hence the error.

### invalid_snippet_arguments

```
A snippet function was passed invalid arguments. Snippets should only be instantiated via `{@render ...}`
```

### lifecycle_outside_component

```
`%name%(...)` can only be used during component initialisation
```

Certain lifecycle methods can only be used during component initialisation. To fix this, make sure you're invoking the method inside the _top level of the instance script_ of your component.

```svelte
<script>
    import { onMount } from 'svelte';

    function handleClick() {
        // This is wrong
        onMount(() => {})
    }

    // This is correct
    onMount(() => {})
</script>

<button onclick={handleClick}>click me</button>
```

### snippet_without_render_tag

```
Attempted to render a snippet without a `{@render}` block. This would cause the snippet code to be stringified instead of its content being rendered to the DOM. To fix this, change `{snippet}` to `{@render snippet()}`.
```

A component throwing this error will look something like this (`children` is not being rendered):

```svelte
<script>
    let { children } = $props();
</script>

{children}
```

...or like this (a parent component is passing a snippet where a non-snippet value is expected):

```svelte
<!--- file: Parent.svelte --->
<ChildComponent>
  {#snippet label()}
    <span>Hi!</span>
  {/snippet}
</ChildComponent>
```

```svelte
<!--- file: Child.svelte --->
<script>
  let { label } = $props();
</script>

<!-- This component doesn't expect a snippet, but the parent provided one -->
<p>{label}</p>
```

### store_invalid_shape

```
`%name%` is not a store with a `subscribe` method
```

### svelte_element_invalid_this_value

```
The `this` prop on `<svelte:element>` must be a string, if defined
```