### 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() } = $props()` ``` ### component_api_changed ``` Calling `%method%` on a component instance (of %component%) is no longer valid in Svelte 5 ``` See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information. ### component_api_invalid_new ``` Attempted to instantiate %component% with `new %name%`, which is no longer valid in Svelte 5. If this component is not under your control, set the `compatibility.componentApi` compiler option to `4` to keep it working. ``` See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information. ### derived_references_self ``` A derived value cannot reference itself recursively ``` ### each_key_duplicate ``` Keyed each block has duplicate key at indexes %a% and %b% ``` ``` Keyed each block has duplicate key `%value%` at indexes %a% and %b% ``` ### each_key_volatile ``` Keyed each block has key that is not idempotent — the key for item at index %index% was `%a%` but is now `%b%`. Keys must be the same each time for a given item ``` The key expression in a keyed each block must return the same value when called multiple times for the same item. Using expressions like `[item.a, item.b]` creates a new array each time, which will never be equal to itself. Instead, use a primitive value or create a stable key like `item.a + '-' + item.b`. ### effect_in_teardown ``` `%rune%` cannot be used inside an effect cleanup function ``` ### effect_in_unowned_derived ``` Effect cannot be created inside a `$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) ``` Effects can only be created while a parent effect is running. This means that they cannot, for example, be created inside an event handler or after an `await` expression (unless the `await` occurs directly inside a component's ` ``` ### 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

{count} is even: {even}

{count} is odd: {odd}

``` This is forbidden because it introduces instability: if `

{count} is even: {even}

` 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. ### svelte_boundary_reset_onerror ``` A `` `reset` function cannot be called while an error is still being handled ``` If a [``](https://svelte.dev/docs/svelte/svelte-boundary) has an `onerror` function, it must not call the provided `reset` function synchronously since the boundary is still in a broken state. Typically, `reset()` is called later, once the error has been resolved. If it's possible to resolve the error inside the `onerror` callback, you must at least wait for the boundary to settle before calling `reset()`, for example using [`tick`](https://svelte.dev/docs/svelte/lifecycle-hooks#tick): ```svelte { fixTheError(); +++await tick();+++ reset(); }}> ```