svelte

Svelte

Reactivity is explicit, compiler-driven, and minimal-runtime. Every reactive declaration uses a $ rune. The compiler transforms declarative code into surgical DOM updates -- no virtual DOM, no diffing, no hidden magic. References contain extended examples, rationale, and edge cases for each topic.

References

• Runes — [${CLAUDE_SKILL_DIR}/references/runes.md]: $state, $derived, $effect, $props, $bindable details
• Components — [${CLAUDE_SKILL_DIR}/references/components.md]: Snippets, events, context, special elements
• SvelteKit — [${CLAUDE_SKILL_DIR}/references/sveltekit.md]: Routing, load functions, form actions, hooks, imports

Runes

$state

• Every mutable reactive value must use $state or $state.raw. Plain let declarations are not reactive.
• Arrays and plain objects become deeply reactive proxies. Mutations trigger granular updates.
• Destructuring $state objects breaks reactivity -- destructured values are snapshots, not live references.
• Use $state on class fields or as first assignment in constructor. The compiler transforms these into getter/setter pairs. Use arrow functions to preserve this in event handlers on classes.
• $state.raw opts out of deep reactivity -- state can only be reassigned, not mutated. Use for large arrays/objects you replace wholesale to avoid proxy overhead.
• $state.snapshot(value) takes a static copy of a reactive proxy for external APIs that don't expect proxies (e.g., structuredClone, logging).
• Import reactive Set, Map, Date, URL from svelte/reactivity when you need reactive built-in types.

Sharing State Across Modules

Cannot directly export reassignable $state. Two patterns:

• Object property (preferred): export $state({ count: 0 }) as a const, mutate properties, export modifier functions.
• Getter function: keep $state private, export getCount() and increment().

Runes only work in .svelte and .svelte.js/.svelte.ts files.

$derived

• Use $derived for all computed values -- never synchronize state with $effect.
• $derived.by(() => { ... }) for complex derivations needing a function body.
• Only synchronously read values are tracked. Use untrack to exempt specific reads.
• Derived values can be temporarily overridden (useful for optimistic UI) -- reverts to derived computation on next dependency update.
• Destructured $derived values are individually reactive.
• Push-pull reactivity: dependents are notified immediately (push) but only recalculated on read (pull). If new value is referentially identical, downstream updates are skipped.

$effect

• $effect is an escape hatch. Use only for side effects: DOM manipulation, analytics, third-party library calls, timers.
• Return a cleanup function when acquiring resources (intervals, listeners).
• Only synchronously read values are tracked -- values read after await or inside setTimeout are NOT tracked.
• Conditional reads: only values read in the last execution are dependencies.
• Runs only in the browser, after DOM updates.
• $effect.pre runs before DOM updates -- use for pre-DOM manipulation like autoscrolling.
• $effect.tracking() returns true if code is running inside a tracking context.
• $effect.root(() => { ... }) creates a non-tracked scope for manual effect lifecycle control. Returns a destroy function.

Never use $effect to synchronize state -- use $derived with callback event handlers or function bindings instead.

$props

• Always destructure props: let { name, count = 0 } = $props().
• Type with an interface in TypeScript: let { name }: Props = $props().
• Renaming: let { class: klass } = $props().
• Rest props: let { a, b, ...rest } = $props().
• All props: let props = $props().
• Unique ID: $props.id() -- consistent across SSR/hydration.
• Props can be temporarily overridden by child. Do NOT mutate prop objects unless $bindable. Use callback props to communicate changes upward.

$bindable

• Marks a prop as two-way bindable: let { value = $bindable() } = $props().
• Parent optionally uses bind:value={variable}.
• Use sparingly -- overuse makes data flow unpredictable. Prefer callback props for most parent-child communication.

$inspect

• Development-only debugging rune. Re-runs when arguments change. Noop in production.
• $inspect(count, message) logs when tracked values change.
• $inspect(value).with((type, ...args) => { ... }) replaces default console.log with custom callback. Type is "init" or "update".
• $inspect.trace() traces which reactive state caused a re-execution. Must be first statement in a function body.

$host

Only available inside custom elements. Provides access to the host element for dispatching custom events.

Components

Structure Order

1. Imports
2. Props ($props())
3. State ($state)
4. Derived values ($derived)
5. Effects ($effect, sparingly)
6. Functions
7. Markup (template)
8. Styles (<style>)

Naming

• Capitalize component names: <MyComponent />. Required for dynamic rendering.
• Component names must be capitalized or use dot notation (item.component).
• Components are dynamic by default -- <svelte:component> is unnecessary. Just use <Thing /> where Thing is a reactive variable.

Events

• Use standard event attributes: onclick={handler}, never on:click={handler}.
• Event attributes are case sensitive -- onclick listens to click, onClick listens to Click.
• No event modifiers -- call event.preventDefault() / event.stopPropagation() in the handler. For capture, append to event name: onclickcapture={...}.
• Callback props for component events -- pass functions as props: let { onEvent } = $props(). Never use createEventDispatcher.
• Event forwarding: accept callback props and spread them onto elements.
• Multiple handlers: combine in a single function (no duplicate attributes).
• Svelte uses event delegation for common events (click, input, keydown) -- single listener at app root. When manually dispatching events, set { bubbles: true }. Prefer on from svelte/events over raw addEventListener.

Snippets

• Use {@render children?.()} for default content. Never use <slot />.
• Named snippets: declare with {#snippet header()}...{/snippet} in parent, accept as props, render with {@render header()}.
• Snippets with parameters pass data from child to parent: {@render item(entry)} in child, {#snippet item(text)} in parent.
• Optional snippets: use {@render children?.()} or {#if children} with fallback.
• Snippets follow lexical scoping -- visible within their declaring block and children.
• Top-level snippets can be exported from <script module> for cross-component use.
• Type snippets with Snippet and Snippet<[ParamType]> from svelte.

Template Syntax

Control flow:

• {#if} / {:else if} / {:else} / {/if} for conditional blocks.
• {#each items as item, index (item.id)} with key expression for lists. Always provide a key for lists that can change. :else renders when array is empty.
• {#key value} destroys and recreates contents when value changes -- triggers entry transitions or resets component state.
• {#await promise} / {:then value} / {:catch error} for async. Short forms: {#await promise then value} skips loading state.

Special tags:

• {@html rawHtml} -- render raw HTML (escape user input to prevent XSS).
• {@const x = expr} -- declare local constant inside a block scope.
• {@debug var1, var2} -- trigger debugger when values change.
• {@render snippet()} -- render a snippet.
• {@attach action} -- attach an action to an element.

Text expressions: {expression} outputs stringified, escaped value. null and undefined are omitted.

Conditional classes: object syntax like clsx: class={{ cool, lame: !cool }}.

Context

• setContext(key, value) / getContext(key) passes data through the component tree without prop drilling.
• Type-safe context: use createContext<T>() from svelte which returns [getContext, setContext] pair.
• Do NOT reassign the context object -- mutate its properties instead.
• For SSR safety, prefer context over global module state.
• Pass functions into setContext to maintain reactivity across boundaries.

Special Elements

• <svelte:boundary> -- error boundary. Use {#snippet failed(error, reset)} for error UI and {#snippet pending()} for loading state with await expressions.
• <svelte:window> -- bind to window events and properties (bind:scrollY).
• <svelte:head> -- insert elements into document.head (SEO meta tags, title).
• <svelte:element this={tag}> -- render a dynamic HTML element.
• <svelte:options> -- set compiler options (customElement, namespace).

Component Instantiation

Components are functions, not classes:

• mount(Component, { target }) for client-side mounting.
• unmount(app) to destroy.
• hydrate instead of mount for server-rendered HTML.

State Management

• No shared module state on the server -- module-level $state is shared across requests during SSR. Use context or event.locals instead.
• Return data from load, don't write to globals. No side effects in load functions.
• Context for SSR-safe shared state -- setContext/getContext for data that must not leak between requests.
• Use $derived for reactive computed values in components -- plain assignments in <script> run once, not reactively.
• Store filter/sort state in URL for survival across reload.
• Use snapshots for ephemeral UI state that should survive back/forward navigation.

File Conventions

• .svelte.js / .svelte.ts for reactive modules -- runes only work in .svelte and .svelte.js/.svelte.ts files.
• $lib for shared code -- import from $lib/ instead of relative paths climbing multiple levels.

SvelteKit

Route Files

• +page.svelte — Page component (receives data from load)
• +page.js — Universal load (server + browser)
• +page.server.js — Server-only load + form actions
• +layout.svelte — Layout wrapper (must render {@render children()})
• +layout.js — Layout universal load
• +layout.server.js — Layout server load
• +error.svelte — Error boundary
• +server.js — API endpoint (GET, POST, etc.)

Key rules: all files can run on the server. All run on the client except +server files. +layout and +error apply to subdirectories too.

Load Functions

Decision tree:

Need	Use
Database, private keys	+page.server.js (PageServerLoad)
Non-serializable return values	+page.js (PageLoad)
External API, no secrets	+page.js (PageLoad)
Both	Both (server data flows to universal)

Universal vs server:

Aspect	Universal (+page.js)	Server (+page.server.js)
Runs on	Server (SSR) + Browser	Server only
Access	params, url, fetch	+ cookies, locals, request
Returns	Any value (classes, components)	Serializable data only

• Use the provided fetch, not global fetch -- inherits cookies, makes relative requests work on server, bypasses HTTP overhead for internal requests.
• Export page options from +page.js: prerender, ssr, csr.
• Layout load data is available to all child pages.
• Stream non-essential data by returning un-awaited promises.
• SvelteKit tracks load dependencies and only reruns when: params change, url properties change, parent() was called and parent reran, or invalidate()/invalidateAll() called.
• Use error() and redirect() from @sveltejs/kit for error and redirect responses.

Form Actions

Server-only POST handlers in +page.server.js. Work without JavaScript.

• Default action: export const actions = { default: async ({ request }) => { ... } }.
• Named actions: action="?/login" on form, multiple actions in the actions object.
• Validation: return fail(400, { field, missing: true }) from action. Access via form prop in the page component.
• Progressive enhancement: add use:enhance from $app/forms for JS-enhanced submission without full page reload.

API Routes

Export HTTP verb handlers from +server.js: GET, POST, PUT, PATCH, DELETE. Return json() or new Response().

Hooks

Server hooks (src/hooks.server.js):

• handle({ event, resolve }) -- intercept every request. Set event.locals, modify response headers.
• handleFetch -- modify server-side fetch calls.
• handleError -- log and sanitize unexpected errors.
• init -- run once at server startup.

Client hooks (src/hooks.client.js):

• handleError -- client-side error handling.

Universal hooks (src/hooks.js):

• reroute -- rewrite URLs before routing.
• transport -- serialize/deserialize custom types across server/client boundary.

Key Imports

Most-used modules: $app/navigation (goto, invalidate), $app/state (page, navigating), $app/forms (enhance), $env/static/private and $env/static/public for environment variables, $lib for shared code. Full imports table in ${CLAUDE_SKILL_DIR}/references/sveltekit.md.

Performance

• Use server load functions to avoid browser-to-API waterfalls.
• Stream non-essential data with un-awaited promises.
• Use $derived instead of $effect for computed values.
• Use link preloading (default on <body>).
• Minimize third-party scripts.
• Use @sveltejs/enhanced-img for image optimization.
• Use dynamic import() for conditional code.
• Deploy frontend near backend to minimize latency.

Application

When writing Svelte code:

• Apply all conventions silently -- don't narrate each rule.
• Always use runes, event attributes, and snippets.
• If an existing codebase uses outdated patterns, follow the codebase and flag the divergence once.
• Type props with interfaces in TypeScript projects.

When reviewing Svelte code:

• Cite the specific violation and show the fix inline.
• Don't lecture -- state what's wrong and how to fix it.

Integration

This skill provides Svelte-specific conventions. The coding skill governs workflow; for TypeScript projects, the typescript skill handles language-level choices; for CSS concerns, the css skill handles styling conventions.