option a

` is removed) - `

cell

` will result in `

cell

` (a `` is auto-inserted) This code will work when the component is rendered on the client (which is why this is a warning rather than an error), but if you use server rendering it will cause hydration to fail. ### non_reactive_update ``` `%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates ``` This warning is thrown when the compiler detects the following: - a variable was declared without `$state` or `$state.raw` - the variable is reassigned - the variable is read in a reactive context In this case, changing the value will not correctly trigger updates. Example: ```svelte

This value updates: {reactive}

This value does not update: {stale}

{ stale = 'updated'; reactive = 'updated'; }}>update ``` To fix this, wrap your variable declaration with `$state`. ### options_deprecated_accessors ``` The `accessors` option has been deprecated. It will have no effect in runes mode ``` ### options_deprecated_immutable ``` The `immutable` option has been deprecated. It will have no effect in runes mode ``` ### options_missing_custom_element ``` The `customElement` option is used when generating a custom element. Did you forget the `customElement: true` compile option? ``` ### options_removed_enable_sourcemap ``` The `enableSourcemap` option has been removed. Source maps are always generated now, and tooling can choose to ignore them ``` ### options_removed_hydratable ``` The `hydratable` option has been removed. Svelte components are always hydratable now ``` ### options_removed_loop_guard_timeout ``` The `loopGuardTimeout` option has been removed ``` ### options_renamed_ssr_dom ``` `generate: "dom"` and `generate: "ssr"` options have been renamed to "client" and "server" respectively ``` ### perf_avoid_inline_class ``` Avoid 'new class' — instead, declare the class at the top level scope ``` ### perf_avoid_nested_class ``` Avoid declaring classes below the top level scope ``` ### reactive_declaration_invalid_placement ``` Reactive declarations only exist at the top level of the instance script ``` ### reactive_declaration_module_script_dependency ``` Reassignments of module-level declarations will not cause reactive statements to update ``` ### script_context_deprecated ``` `context="module"` is deprecated, use the `module` attribute instead ``` ```svelte ``` ### script_unknown_attribute ``` Unrecognized attribute — should be one of `generics`, `lang` or `module`. If this exists for a preprocessor, ensure that the preprocessor removes it ``` ### slot_element_deprecated ``` Using `` to render parent content is deprecated. Use `{@render ...}` tags instead ``` See [the migration guide](v5-migration-guide#Snippets-instead-of-slots) for more info. ### state_referenced_locally ``` State referenced in its own scope will never update. Did you mean to reference it inside a closure? ``` This warning is thrown when the compiler detects the following: - A reactive variable is declared - the variable is reassigned - the variable is referenced inside the same scope it is declared and it is a non-reactive context In this case, the state reassignment will not be noticed by whatever you passed it to. For example, if you pass the state to a function, that function will not notice the updates: ```svelte count++}> increment ``` ```svelte

The count is {count}

``` To fix this, reference the variable such that it is lazily evaluated. For the above example, this can be achieved by wrapping `count` in a function: ```svelte count++}> increment ``` ```svelte

The count is {+++count()+++}

``` For more info, see [Passing state into functions]($state#Passing-state-into-functions). ### store_rune_conflict ``` It looks like you're using the `$%name%` rune, but there is a local binding called `%name%`. Referencing a local variable with a `$` prefix will create a store subscription. Please rename `%name%` to avoid the ambiguity ``` ### svelte_component_deprecated ``` `` is deprecated in runes mode — components are dynamic by default ``` In previous versions of Svelte, the component constructor was fixed when the component was rendered. In other words, if you wanted `` to re-render when `X` changed, you would either have to use `` or put the component inside a `{#key X}...{/key}` block. In Svelte 5 this is no longer true — if `X` changes, `` re-renders. In some cases `` syntax can be used as a replacement; a lowercased variable with property access is recognized as a component in Svelte 5. For complex component resolution logic, an intermediary, capitalized variable may be necessary. E.g. in places where `@const` can be used: ```svelte {#each items as item} ------ +++{@const Component = item.condition ? Y : Z}+++ ++++++ {/each} ``` A derived value may be used in other contexts: ```svelte ------ ++++++ ``` ### svelte_element_invalid_this ``` `this` should be an `{expression}`. Using a string attribute value will cause an error in future versions of Svelte ``` ### svelte_self_deprecated ``` `` is deprecated — use self-imports (e.g. `import %name% from './%basename%'`) instead ``` See [the note in the docs](legacy-svelte-self) for more info. ### unknown_code ``` `%code%` is not a recognised code ``` ``` `%code%` is not a recognised code (did you mean `%suggestion%`?) ``` # Runtime errors ## Client errors ### 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() } = $props()` ``` ### component_api_changed ``` %parent% called `%method%` on an instance of %component%, which is no longer valid in Svelte 5 ``` See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information. ### component_api_invalid_new ``` Attempted to instantiate %component% with `new %name%`, which is no longer valid in Svelte 5. If this component is not under your control, set the `compatibility.componentApi` compiler option to `4` to keep it working. ``` See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information. ### derived_references_self ``` A derived value cannot reference itself recursively ``` ### each_key_duplicate ``` Keyed each block has duplicate key at indexes %a% and %b% ``` ``` Keyed each block has duplicate key `%value%` at indexes %a% and %b% ``` ### effect_in_teardown ``` `%rune%` cannot be used inside an effect cleanup function ``` ### effect_in_unowned_derived ``` Effect cannot be created inside a `$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_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 ``` ### 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 `$props()` such as `%property%` are readonly ``` ### rune_outside_svelte ``` The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files ``` ### state_descriptors_fixed ``` Property descriptors defined on `$state` objects must contain `value` and always be `enumerable`, `configurable` and `writable`. ``` ### state_prototype_fixed ``` Cannot set prototype of `$state` object ``` ### state_unsafe_local_read ``` Reading state that was created inside the same derived is forbidden. Consider using `untrack` to read locally created state ``` ### state_unsafe_mutation ``` Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state` ``` ## Server errors ### 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 ### 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 {entry} ``` ```svelte

{#each items as item}
• {@render children(item)}
{/each}

``` 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. ### 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 click me ``` ### store_invalid_shape ``` `%name%` is not a store with a `subscribe` method ``` ### svelte_element_invalid_this_value ``` The `this` prop on `` must be a string, if defined ``` # Runtime warnings ## Client warnings ### assignment_value_stale ``` Assignment to `%property%` property (%location%) will evaluate to the right-hand side, not the value of `%property%` following the assignment. This may result in unexpected behaviour. ``` Given a case like this... ```svelte add

items: {JSON.stringify(object.items)}

``` ...the array being pushed to when the button is first clicked is the `[]` on the right-hand side of the assignment, but the resulting value of `object.array` is an empty state proxy. As a result, the pushed value will be discarded. You can fix this by separating it into two statements: ```js let object = { array: [0] }; // ---cut--- function add() { object.array ??= []; object.array.push(object.array.length); } ``` ### binding_property_non_reactive ``` `%binding%` is binding to a non-reactive property ``` ``` `%binding%` (%location%) is binding to a non-reactive property ``` ### console_log_state ``` Your `console.%method%` contained `$state` proxies. Consider using `$inspect(...)` or `$state.snapshot(...)` instead ``` When logging a [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy), browser devtools will log the proxy itself rather than the value it represents. In the case of Svelte, the 'target' of a `$state` proxy might not resemble its current value, which can be confusing. The easiest way to log a value as it changes over time is to use the [`$inspect`](/docs/svelte/$inspect) rune. Alternatively, to log things on a one-off basis (for example, inside an event handler) you can use [`$state.snapshot`](/docs/svelte/$state#$state.snapshot) to take a snapshot of the current value. ### event_handler_invalid ``` %handler% should be a function. Did you mean to %suggestion%? ``` ### hydration_attribute_changed ``` The `%attribute%` attribute on `%html%` changed its value between server and client renders. The client value, `%value%`, will be ignored in favour of the server value ``` Certain attributes like `src` on an `` element will not be repaired during hydration, i.e. the server value will be kept. That's because updating these attributes can cause the image to be refetched (or in the case of an `