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 ``` This reference only captures the initial value of `%name%`. Did you mean to reference it inside a %type% instead? ``` This warning is thrown when the compiler detects the following: - A reactive variable is declared - ...and later reassigned... - ...and referenced in the same scope This 'breaks the link' to the original state declaration. For example, if you pass the state to a function, the function loses access to the state once it is reassigned: ```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 `$` prefix will create a store subscription. Please rename `%name%` to avoid the ambiguity ``` ### svelte_component_deprecated ``` `` is deprecated in runes mode — components are dynamic by default ``` In previous versions of Svelte, the component constructor was fixed when the component was rendered. In other words, if you wanted `` to re-render when `X` changed, you would either have to use `` or put the component inside a `{#key X}...{/key}` block. In Svelte 5 this is no longer true — if `X` changes, `` re-renders. In some cases `` syntax can be used as a replacement; a lowercased variable with property access is recognized as a component in Svelte 5. For complex component resolution logic, an intermediary, capitalized variable may be necessary. E.g. in places where `@const` can be used: ```svelte {#each items as item} ------ +++{@const Component = item.condition ? Y : Z}+++ ++++++ {/each} ``` A derived value may be used in other contexts: ```svelte ------ ++++++ ``` ### svelte_element_invalid_this ``` `this` should be an `{expression}`. Using a string attribute value will cause an error in future versions of Svelte ``` ### svelte_self_deprecated ``` `` is deprecated — use self-imports (e.g. `import %name% from './%basename%'`) instead ``` See [the note in the docs](legacy-svelte-self) for more info. ### unknown_code ``` `%code%` is not a recognised code ``` ``` `%code%` is not a recognised code (did you mean `%suggestion%`?) ``` # Runtime errors ## Client errors ### async_derived_orphan ``` Cannot create a `$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) ``` ### effect_pending_outside_reaction ``` `$effect.pending()` can only be called inside an effect or derived ``` ### effect_update_depth_exceeded ``` Maximum update depth exceeded. This typically indicates that an effect reads and writes the same piece of state ``` If an effect updates some state that it also depends on, it will re-run, potentially in a loop: ```js let count = $state(0); $effect(() => { // this both reads and writes `count`, // so will run in an infinite loop count += 1; }); ``` (Svelte intervenes before this can crash your browser tab.) The same applies to array mutations, since these both read and write to the array: ```js let array = $state(['hello']); $effect(() => { array.push('goodbye'); }); ``` Note that it's fine for an effect to re-run itself as long as it 'settles': ```js let array = ['a', 'b', 'c']; // ---cut--- $effect(() => { // this is okay, because sorting an already-sorted array // won't result in a mutation array.sort(); }); ``` Often when encountering this issue, the value in question shouldn't be state (for example, if you are pushing to a `logs` array in an effect, make `logs` a normal array rather than `$state([])`). In the rare cases where you really _do_ need to write to state in an effect — [which you should avoid]($effect#When-not-to-use-$effect) — you can read the state with [untrack](svelte#untrack) to avoid adding it as a dependency. ### 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. ### fork_discarded ``` Cannot commit a fork that was already discarded ``` ### fork_timing ``` Cannot create a fork inside an effect or when state changes are pending ``` ### get_abort_signal_outside_reaction ``` `getAbortSignal()` can only be called inside an effect or derived ``` ### hydratable_missing_but_required ``` Expected to find a hydratable with key `%key%` during hydration, but did not. ``` This can happen if you render a hydratable on the client that was not rendered on the server, and means that it was forced to fall back to running its function blockingly during hydration. This is bad for performance, as it blocks hydration until the asynchronous work completes. ```svelte ``` ### 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++}>{count}

{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(); }}> ``` ## Server errors ### async_local_storage_unavailable ``` The node API `AsyncLocalStorage` is not available, but is required to use async server rendering. ``` Some platforms require configuration flags to enable this API. Consult your platform's documentation. ### await_invalid ``` Encountered asynchronous work while rendering synchronously. ``` You (or the framework you're using) called [`render(...)`](svelte-server#render) with a component containing an `await` expression. Either `await` the result of `render` or wrap the `await` (or the component containing it) in a [``](svelte-boundary) with a `pending` snippet. ### dynamic_element_invalid_tag ``` `` is not a valid element name — the element will not be rendered ``` The value passed to the `this` prop of `` must be a valid HTML element, SVG element, MathML element, or custom element name. A value containing invalid characters (such as whitespace or special characters) was provided, which could be a security risk. Ensure only valid tag names are passed. ### html_deprecated ``` The `html` property of server render results has been deprecated. Use `body` instead. ``` ### hydratable_clobbering ``` Attempted to set `hydratable` with key `%key%` twice with different values. %stack% ``` This error occurs when using `hydratable` multiple times with the same key. To avoid this, you can: - Ensure all invocations with the same key result in the same value - Update the keys to make both instances unique ```svelte ``` ### hydratable_serialization_failed ``` Failed to serialize `hydratable` data for key `%key%`. `hydratable` can serialize anything [`uneval` from `devalue`](https://npmjs.com/package/uneval) can, plus Promises. Cause: %stack% ``` ### invalid_csp ``` `csp.nonce` was set while `csp.hash` was `true`. These options cannot be used simultaneously. ``` ### 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. ### server_context_required ``` Could not resolve `render` context. ``` Certain functions such as `hydratable` cannot be invoked outside of a `render(...)` call, such as at the top level of a module. ## Shared errors ### experimental_async_required ``` Cannot use `%name%(...)` unless the `experimental.async` compiler option is `true` ``` ### 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. ### 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 click me ``` ### missing_context ``` Context was not set in a parent component ``` The [`createContext()`](svelte#createContext) utility returns a `[get, set]` pair of functions. `get` will throw an error if `set` was not used to set the context in a parent component. ### 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 {children} ``` ...or like this (a parent component is passing a snippet where a non-snippet value is expected): ```svelte {#snippet label()} Hi! {/snippet} ``` ```svelte

{label}

``` ### 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); } ``` ### await_reactivity_loss ``` Detected reactivity loss when reading `%name%`. This happens when state is read in an async function after an earlier `await` ``` Svelte's signal-based reactivity works by tracking which bits of state are read when a template or `$derived(...)` expression executes. If an expression contains an `await`, Svelte transforms it such that any state _after_ the `await` is also tracked — in other words, in a case like this... ```js let a = Promise.resolve(1); let b = 2; // ---cut--- let total = $derived(await a + b); ``` ...both `a` and `b` are tracked, even though `b` is only read once `a` has resolved, after the initial execution. This does _not_ apply to an `await` that is not 'visible' inside the expression. In a case like this... ```js let a = Promise.resolve(1); let b = 2; // ---cut--- async function sum() { return await a + b; } let total = $derived(await sum()); ``` ...`total` will depend on `a` (which is read immediately) but not `b` (which is not). The solution is to pass the values into the function: ```js let a = Promise.resolve(1); let b = 2; // ---cut--- /** * @param {Promise} a * @param {number} b */ async function sum(a, b) { return await a + b; } let total = $derived(await sum(a, b)); ``` ### await_waterfall ``` An async derived, `%name%` (%location%) was not read immediately after it resolved. This often indicates an unnecessary waterfall, which can slow down your app ``` In a case like this... ```js async function one() { return 1 } async function two() { return 2 } // ---cut--- let a = $derived(await one()); let b = $derived(await two()); ``` ...the second `$derived` will not be created until the first one has resolved. Since `await two()` does not depend on the value of `a`, this delay, often described as a 'waterfall', is unnecessary. (Note that if the values of `await one()` and `await two()` subsequently change, they can do so concurrently — the waterfall only occurs when the deriveds are first created.) You can solve this by creating the promises first and _then_ awaiting them: ```js async function one() { return 1 } async function two() { return 2 } // ---cut--- let aPromise = $derived(one()); let bPromise = $derived(two()); let a = $derived(await aPromise); let b = $derived(await bPromise); ``` ### binding_property_non_reactive ``` `%binding%` is binding to a non-reactive property ``` ``` `%binding%` (%location%) is binding to a non-reactive property ``` ### console_log_state ``` Your `console.%method%` contained `$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%? ``` ### hydratable_missing_but_expected ``` Expected to find a hydratable with key `%key%` during hydration, but did not. ``` This can happen if you render a hydratable on the client that was not rendered on the server, and means that it was forced to fall back to running its function blockingly during hydration. This is bad for performance, as it blocks hydration until the asynchronous work completes. ```svelte ``` ### 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 `