## Directories ### error_&_warning_reference Complete reference of all Svelte runtime errors, compiler errors, compiler warnings, and server-side errors with causes and solutions. ## Complete Error and Warning Reference for Svelte Comprehensive documentation of all runtime errors, compiler errors, compiler warnings, and server-side errors in Svelte 5. ### Client-side Runtime Errors **Reactivity Errors**: `async_derived_orphan` (async deriveds need effects), `derived_references_self`, `effect_in_unowned_derived`, `effect_orphan`, `effect_in_teardown`, `effect_pending_outside_reaction`, `effect_update_depth_exceeded` (infinite loops from reading/writing same state - use `untrack()` or regular variables), `state_unsafe_mutation` (cannot mutate state in derived/inspect/templates - make everything derived or use effects). **Binding Errors**: `bind_invalid_checkbox_value` (use `bind:checked`), `bind_invalid_export` (use `bind:this`), `bind_not_bindable` (mark with `$bindable()`), `props_invalid_value` (cannot bind undefined when fallback exists), `props_rest_readonly`. **Component Errors**: `component_api_changed` (methods no longer valid in Svelte 5), `component_api_invalid_new` (cannot use `new` on components). **Loop Errors**: `each_key_duplicate`. **Context & Lifecycle**: `set_context_after_init` (must call during initialization), `lifecycle_legacy_only`. **State Errors**: `state_descriptors_fixed`, `state_prototype_fixed`. **Async/Fork Errors**: `experimental_async_fork`, `fork_discarded`, `fork_timing`, `flush_sync_in_effect`. **Other**: `get_abort_signal_outside_reaction`, `hydration_failed`, `invalid_snippet` (use optional chaining), `rune_outside_svelte`, `svelte_boundary_reset_onerror` (wait with `await tick()` before calling reset). ### Client-side Warnings **State & Reactivity**: `assignment_value_stale` (separate statements), `await_reactivity_loss` (pass values as parameters), `await_waterfall` (create promises first), `console_log_state` (use `$inspect()` or `$state.snapshot()`), `legacy_recursive_reactive_block`, `non_reactive_update` (use `$state()` for updates), `state_proxy_equality_mismatch` (compare values consistently), `state_referenced_locally` (wrap in function for lazy evaluation). **Binding & Props**: `binding_property_non_reactive`, `ownership_invalid_binding` (use `bind:` in parent), `ownership_invalid_mutation` (use `bind:` or `$bindable()`), `state_proxy_unmount` (use `$state.raw()`). **Hydration**: `hydration_attribute_changed` (ensure server/client match or force update in effect), `hydration_html_changed` (same pattern as attribute_changed), `hydration_mismatch` (server HTML structure differs). **Events & Lifecycle**: `event_handler_invalid`, `lifecycle_double_unmount`. **Snippets & Rendering**: `invalid_raw_snippet_render` (must return single element), `svelte_boundary_reset_noop` (only works first time). **Other**: `select_multiple_invalid_value` (must be array/null/undefined), `transition_slide_display` (doesn't work with inline/table/contents). ### Compiler Errors (200+) **Animations**: `animation_duplicate`, `animation_invalid_placement` (only child of keyed each), `animation_missing_key`. **Attributes**: `attribute_contenteditable_dynamic`, `attribute_contenteditable_missing`, `attribute_duplicate`, `attribute_empty_shorthand`, `attribute_invalid_event_handler`, `attribute_invalid_multiple`, `attribute_invalid_type`, `attribute_invalid_sequence_expression` (wrap in parens in runes mode), `attribute_unquoted_sequence`. **Bindings**: `bind_group_invalid_expression`, `bind_group_invalid_snippet_parameter`, `bind_invalid_expression`, `bind_invalid_name`, `bind_invalid_parens`, `bind_invalid_target`, `bind_invalid_value`. **Blocks**: `block_duplicate_clause`, `block_invalid_continuation_placement`, `block_invalid_elseif` (use `else if`), `block_invalid_placement`, `block_unclosed`, `block_unexpected_character`, `block_unexpected_close`. **Const Tags**: `const_tag_cycle`, `const_tag_invalid_expression`, `const_tag_invalid_placement` (must be immediate child of snippet/if/else/each/then/catch/fragment/boundary/component), `const_tag_invalid_reference` (scoping issue - const in boundary not available in nested snippet). **CSS**: `css_empty_declaration`, `css_expected_identifier`, `css_global_block_invalid_combinator`, `css_global_block_invalid_declaration`, `css_global_block_invalid_list` (cannot mix `:global` with non-global in selector list), `css_global_block_invalid_modifier`, `css_global_block_invalid_modifier_start`, `css_global_block_invalid_placement`, `css_global_invalid_placement`, `css_global_invalid_selector`, `css_global_invalid_selector_list`, `css_nesting_selector_invalid_placement`, `css_selector_invalid`, `css_type_selector_invalid_placement`. **Declarations**: `declaration_duplicate`, `declaration_duplicate_module_import`, `constant_assignment`, `constant_binding`. **Each Blocks**: `each_item_invalid_assignment` (cannot reassign each argument in runes mode - use index instead: `{#each array as entry, i}` then `array[i] = value`), `each_key_without_as`. **Elements**: `element_invalid_closing_tag`, `element_invalid_closing_tag_autoclosed`, `element_unclosed`. **Event Handlers**: `event_handler_invalid_component_modifier` (only `once` on components), `event_handler_invalid_modifier`, `event_handler_invalid_modifier_combination`. **Exports**: `export_undefined`, `export_let_unused`, `derived_invalid_export` (export function instead). **Experimental**: `experimental_async` (need `experimental.async` compiler option for await in deriveds/templates/top-level). **Global**: `global_reference_invalid` (use `globalThis.%name%`). **Host**: `host_invalid_placement` (`$host()` only in custom element instances). **Imports**: `import_svelte_internal_forbidden`. **Inspect**: `inspect_trace_generator`, `inspect_trace_invalid_placement` (must be first statement). **Invalid/Parse**: `invalid_arguments_usage`, `js_parse_error`. **Legacy**: `legacy_await_invalid`, `legacy_export_invalid` (use `$props()`), `legacy_props_invalid` (use `$props()`), `legacy_reactive_statement_invalid` (use `$derived`/`$effect`), `legacy_rest_props_invalid`. **Misc**: `mixed_event_handler_syntaxes`, `module_illegal_default_export`, `node_invalid_placement` (HTML repair breaks Svelte - examples: `

` becomes `

`, `` becomes ``, `

` becomes `

`). **Options**: `options_invalid_value`, `options_removed`, `options_unrecognised`. **Props**: `props_duplicate` (cannot use `$props()` twice), `props_id_invalid_placement` (only at top level), `props_illegal_name` (cannot start with `$$`), `props_invalid_identifier` (only object destructuring), `props_invalid_pattern` (no nested properties/computed keys), `props_invalid_placement`. **Runes**: `rune_invalid_arguments`, `rune_invalid_arguments_length`, `rune_invalid_computed_property`, `rune_invalid_name`, `rune_invalid_spread`, `rune_invalid_usage`, `rune_missing_parentheses`, `rune_removed`, `rune_renamed`. **Scripts**: `script_duplicate`, `script_invalid_attribute_value`, `script_invalid_context`, `script_reserved_attribute`, `bindable_invalid_location` (`$bindable()` only in `$props()`). **Slots**: `slot_attribute_duplicate`, `slot_attribute_invalid`, `slot_attribute_invalid_placement`, `slot_default_duplicate`, `slot_element_invalid_attribute`, `slot_element_invalid_name`, `slot_element_invalid_name_default`, `slot_snippet_conflict`. **Snippets**: `snippet_conflict` (cannot use `` and `{@render}` together), `snippet_invalid_export` (only references module-level script or other exportable snippets), `snippet_invalid_rest_parameter` (use array), `snippet_parameter_assignment`, `snippet_shadowing_prop`. **State**: `state_field_duplicate`, `state_field_invalid_assignment`, `state_invalid_export` (export function or only mutate properties), `state_invalid_placement` (only as variable/class field/constructor assignment). **Stores**: `store_invalid_scoped_subscription`, `store_invalid_subscription`, `store_invalid_subscription_module`. **Styles**: `style_directive_invalid_modifier` (only `important`), `style_duplicate`. **Svelte Elements**: `svelte_body_illegal_attribute`, `svelte_boundary_invalid_attribute`, `svelte_boundary_invalid_attribute_value`, `svelte_component_invalid_this`, `svelte_component_missing_this`, `svelte_element_missing_this`, `svelte_fragment_invalid_attribute`, `svelte_fragment_invalid_placement`, `svelte_head_illegal_attribute`, `svelte_meta_duplicate`, `svelte_meta_invalid_content`, `svelte_meta_invalid_placement`, `svelte_meta_invalid_tag`, `svelte_options_deprecated_tag`, `svelte_options_invalid_attribute`, `svelte_options_invalid_attribute_value`, `svelte_options_invalid_customelement`, `svelte_options_invalid_customelement_props`, `svelte_options_invalid_customelement_shadow`, `svelte_options_invalid_tagname`, `svelte_options_reserved_tagname`, `svelte_options_unknown_attribute`, `svelte_self_invalid_placement` (only in if/each/snippet blocks or slots). **Tags**: `tag_invalid_name`, `tag_invalid_placement`, `textarea_invalid_content`, `title_illegal_attribute`, `title_invalid_content`. **Transitions**: `transition_conflict`, `transition_duplicate`. **TypeScript**: `typescript_invalid_feature` (use preprocessor). **Misc**: `unexpected_eof`, `unexpected_reserved_word`, `unterminated_string_constant`, `void_element_invalid_content`. ### Compiler Warnings (100+) **Accessibility (a11y_*)**: `accesskey`, `aria_activedescendant_has_tabindex`, `aria_attributes` (reserved elements), `autofocus`, `click_events_have_key_events` (need keyboard handlers), `consider_explicit_label`, `distracting_elements`, `figcaption_index`, `figcaption_parent`, `hidden` (certain elements), `img_redundant_alt`, `incorrect_aria_attribute_type` (aria-hidden: boolean, aria-activedescendant: ID, aria-labelledby: ID list, aria-level: integer, aria-sort: token, aria-pressed: tristate), `interactive_supports_focus`, `invalid_attribute` (href), `label_has_associated_control`, `media_has_caption`, `misplaced_role`, `misplaced_scope`, `missing_attribute` (a: href, area: alt/aria-label/aria-labelledby, html: lang, iframe: title, img: alt, object: title/aria-label/aria-labelledby, input[type=image]: alt/aria-label/aria-labelledby), `missing_content`, `mouse_events_have_key_events`, `no_abstract_role`, `no_interactive_element_to_noninteractive_role`, `no_noninteractive_element_interactions`, `no_noninteractive_element_to_interactive_role`, `no_noninteractive_tabindex`, `no_redundant_roles`, `no_static_element_interactions`, `positive_tabindex`, `role_has_required_aria_props`, `role_supports_aria_props`, `unknown_aria_attribute`, `unknown_role`. **Attributes**: `attribute_avoid_is`, `attribute_global_event_reference`, `attribute_illegal_colon`, `attribute_invalid_property_name`, `attribute_quoted`. **Code Quality**: `bidirectional_control_characters`, `bind_invalid_each_rest`, `block_empty`, `component_name_lowercase`, `css_unused_selector` (use `:global()` for selectors targeting `{@html}` or child components), `custom_element_props_identifier`, `element_implicitly_closed`, `element_invalid_self_closing_tag`, `event_directive_deprecated` (use `on%name%`), `export_let_unused`, `legacy_code`, `legacy_component_creation`, `node_invalid_placement_ssr`, `options_deprecated_accessors`, `options_deprecated_immutable`, `options_missing_custom_element`, `options_removed_enable_sourcemap`, `options_removed_hydratable`, `options_removed_loop_guard_timeout`, `options_renamed_ssr_dom`, `perf_avoid_inline_class`, `perf_avoid_nested_class`, `reactive_declaration_invalid_placement`, `reactive_declaration_module_script_dependency`, `script_context_deprecated` (use `module` attribute), `script_unknown_attribute`, `slot_element_deprecated` (use `{@render}`), `store_rune_conflict`, `svelte_component_deprecated` (use dynamic components), `svelte_element_invalid_this`, `svelte_self_deprecated`, `unknown_code`. ### Server-side Errors **await_invalid**: Async work in sync render - await result or wrap in ``. **html_deprecated**: Use `body` instead of `html` property. **lifecycle_function_unavailable**: Methods like `mount` unavailable on server. ### Shared Errors **invalid_default_snippet**: Cannot use `{@render children(...)}` with `let:` directives - use named snippets. **invalid_snippet_arguments**: Snippets only instantiated via `{@render}`. **lifecycle_outside_component**: Lifecycle methods only at top level of instance script. **missing_context**: Context not set in parent - `get` throws if `set` not called. **snippet_without_render_tag**: Snippet stringified instead of rendered - use `{@render}`. **store_invalid_shape**: Not a store with `subscribe` method. **svelte_element_invalid_this_value**: `this` prop must be string. ### Shared Warnings **dynamic_void_element_content**: Void elements ignore children. **state_snapshot_uncloneable**: `$state.snapshot()` returns original for uncloneable values (DOM elements, etc.). ## Pages ### svelte_api_reference Svelte 5 public API: Component type, mount/hydrate/unmount functions, lifecycle hooks (onMount/onDestroy), context API (createContext/getContext/setContext), event dispatcher, state sync (tick/settled/flushSync), fork for speculative updates, untrack/getAbortSignal utilities, and TypeScript types. ## Core Components and Types **SvelteComponent** - Base class for Svelte 4 components (deprecated in Svelte 5+). Use `Component` type and `mount()` function instead. Had methods like `$destroy()`, `$on()`, `$set()` which are now deprecated. **SvelteComponentTyped** - Typed variant of SvelteComponent (deprecated, use `Component` instead). **Component** - Type for creating strongly typed Svelte components. Example: ```ts import type { Component } from 'svelte'; export declare const MyComponent: Component<{ foo: string }> {} ``` Can be used in TypeScript to provide intellisense and type checking when using components. ## Lifecycle Functions **onMount** - Runs once after component mounts to DOM. Can return cleanup function. Doesn't run during SSR. ```ts onMount(() => { console.log('mounted'); return () => console.log('cleanup'); }); ``` **onDestroy** - Runs before component unmounts. Only lifecycle that runs in SSR. **beforeUpdate** (deprecated, use `$effect.pre`) - Runs before component updates after state changes. **afterUpdate** (deprecated, use `$effect`) - Runs after component updates. ## Context API **createContext** - Type-safe context pair (available since 5.40.0): ```ts const [getTheme, setTheme] = createContext(); // In parent: setTheme('dark') // In child: const theme = getTheme() ``` **getContext** - Retrieves context by key from parent component. **setContext** - Associates context object with key for children to access. **getAllContexts** - Gets entire context map from closest parent. **hasContext** - Checks if key exists in parent context. ## Event Handling **createEventDispatcher** (deprecated, use callback props or `$host()`) - Creates typed event dispatcher: ```ts const dispatch = createEventDispatcher<{ loaded: null; change: string; optional: number | null; }>(); dispatch('change', 'value'); ``` ## Component Mounting **mount** - Mounts component to target, returns exports: ```ts import { mount } from 'svelte'; import App from './App.svelte'; const app = mount(App, { target: document.body, props: { foo: 'bar' } }); ``` **hydrate** - Hydrates component on given target with SSR-rendered HTML. **unmount** - Unmounts component, optionally plays outro transitions: ```ts unmount(app, { outro: true }); ``` ## State and Synchronization **tick** - Returns promise that resolves after pending state changes applied to DOM. **settled** - Returns promise that resolves after state changes and async work complete (available since 5.36). **flushSync** - Synchronously flushes pending updates, optionally runs callback: ```ts flushSync(() => { /* updates applied immediately */ }); ``` **fork** - Creates off-screen fork for speculative state changes (available since 5.42): ```ts const fork = fork(() => { state.value = 'new'; }); // Later: fork.commit() or fork.discard() ``` ## Utilities **untrack** - Prevents state reads inside function from being treated as dependencies: ```ts $effect(() => { save(data, { timestamp: untrack(() => time) }); }); ``` **getAbortSignal** - Returns AbortSignal that aborts when derived/effect re-runs or destroys: ```ts const data = $derived(await fetch('/items', { signal: getAbortSignal() })); ``` **createRawSnippet** - Creates snippet programmatically with render and optional setup functions. ## Types **ComponentProps** - Extracts props type from component: ```ts const props: ComponentProps = { foo: 'bar' }; ``` **ComponentConstructorOptions** (deprecated) - Options for old class-based component constructor. **ComponentEvents** (deprecated) - Extracts events type from SvelteComponent. **ComponentType** (deprecated) - Type for class-based components. **EventDispatcher** - Interface for event dispatcher with typed dispatch method. **Fork** - Interface with `commit()` and `discard()` methods. **MountOptions** - Options for mount function: target, anchor, events, context, intro. **Snippet** - Type for snippet blocks with typed parameters. **ComponentInternals** - Internal implementation details (branded type). ### svelte-action Action and ActionReturn TypeScript interfaces for typing element lifecycle functions with optional update/destroy callbacks and custom attributes/events. ## Action Actions are functions called when an element is created. Type them with the `Action` interface: ```ts export const myAction: Action = (node, param = { someProperty: true }) => { // ... } ``` `Action` and `Action` both mean the action accepts no parameters. Actions can return an object with optional `update` and `destroy` methods, and can specify additional attributes/events via the `Attributes` type parameter. ## ActionReturn Actions can return an object with two optional properties: - `update(parameter)`: Called whenever the action's parameter changes, after Svelte updates the markup - `destroy()`: Called after the element is unmounted You can specify additional attributes and events the action enables: ```ts interface Attributes { newprop?: string; 'on:event': (e: CustomEvent) => void; } export function myAction(node: HTMLElement, parameter: Parameter): ActionReturn { return { update: (updatedParameter) => {...}, destroy: () => {...} }; } ``` Note: Actions have been superseded by attachments. ### svelte-animate flip animation function using FLIP technique to animate element position changes between DOMRect start/end states with configurable delay, duration, and easing ## flip The `flip` function animates element position changes using the FLIP technique (First, Last, Invert, Play). It calculates start and end positions of an element and animates between them by translating x and y values. ```js import { flip } from 'svelte/animate'; function flip( node: Element, { from, to }: { from: DOMRect; to: DOMRect }, params?: FlipParams ): AnimationConfig; ``` ## AnimationConfig Configuration object returned by animation functions with properties: - `delay?: number` - delay before animation starts - `duration?: number` - animation duration in milliseconds - `easing?: (t: number) => number` - easing function - `css?: (t: number, u: number) => string` - returns CSS to apply at each frame - `tick?: (t: number, u: number) => void` - callback invoked at each frame ## FlipParams Parameters for the `flip` function: - `delay?: number` - delay before animation starts - `duration?: number | ((len: number) => number)` - duration in ms or function that calculates it from distance - `easing?: (t: number) => number` - easing function ### attachments API for creating and converting attachments: createAttachmentKey() returns symbol for spreading attachments onto elements; fromAction() converts actions to attachments; Attachment is a mount/unmount lifecycle function. ## createAttachmentKey Creates a symbol key for programmatic attachment creation. When spread onto an element, the symbol key is recognized as an attachment—an alternative to `{@attach ...}` syntax. ```js import { createAttachmentKey } from 'svelte/attachments'; const props = { class: 'cool', onclick: () => alert('clicked'), [createAttachmentKey()]: (node) => { node.textContent = 'attached!'; } }; ``` ```dts function createAttachmentKey(): symbol; ``` ## fromAction Converts an action into an attachment with identical behavior. The second argument must be a function that returns the action's argument, not the argument itself. ```js import { fromAction } from 'svelte/attachments'; // with an action

...

// with an attachment

bar)}>...

``` ```dts function fromAction( action: Action | ((element: E, arg: T) => void | ActionReturn), fn: () => T ): Attachment; function fromAction( action: Action | ((element: E) => void | ActionReturn) ): Attachment; ``` ## Attachment A function that runs when an element mounts to the DOM and optionally returns a cleanup function called on unmount. Can be attached via `{@attach ...}` or by spreading an object with a property created by `createAttachmentKey`. ```dts interface Attachment { (element: T): void | (() => void); } ``` ### svelte-compiler Svelte compiler API: compile/compileModule/parse/preprocess/migrate functions with CompileOptions (name, customElement, css, runes, etc.), ModuleCompileOptions (dev, generate, filename), CompileResult (js/css/warnings/metadata/ast), PreprocessorGroup/Preprocessor/Processed types, and comprehensive AST type definitions. ## Compiler API The `svelte/compiler` module exports functions for programmatically compiling Svelte components and modules. ### Core Functions **VERSION**: Current version string from package.json. **compile(source, options)**: Converts `.svelte` source code into a JavaScript module exporting a component. Returns `CompileResult` with `js`, `css`, `warnings`, `metadata`, and `ast` properties. **compileModule(source, options)**: Compiles JavaScript source containing runes into a JavaScript module. Returns `CompileResult`. **parse(source, options)**: Parses a component and returns its AST. With `modern: true` returns modern AST format; without or `modern: false` returns legacy AST. Options include `filename` and `loose`. **preprocess(source, preprocessor, options)**: Applies preprocessor hooks to transform component source code (e.g., converting ` ``` **custom_element_props_identifier**: Using rest element or non-destructured `$props()` prevents custom element prop inference. **element_implicitly_closed**: HTML elements implicitly closed by another element. Add explicit closing tag. ```svelte

hello

``` **element_invalid_self_closing_tag**: Self-closing tags for non-void elements are ambiguous. Use explicit closing tags. ```svelte some text some text ``` **event_directive_deprecated**: `on:%name%` deprecated, use `on%name%` attribute instead. **export_let_unused**: Unused export property (use `export const` for external reference only). **legacy_code**: Old syntax no longer valid (with suggestions). **legacy_component_creation**: Svelte 5 components not classes; use `mount` or `hydrate` from 'svelte'. **node_invalid_placement_ssr**: HTML structure violation causing browser repair, breaking hydration. Examples: `

` cannot contain block elements, `

`, `` auto-inserts ``. **non_reactive_update**: Variable updated but not declared with `$state()`, won't trigger updates. ```svelte

API Reference