SvelteKit + Svelte 5 Quick Reference (February 2026)

Current Versions

Package	Version	Notes
Svelte	~5.50.x	Svelte 5 is the current stable. Svelte 4 is legacy.
SvelteKit	~2.50.x	SvelteKit 2 is current stable. SvelteKit 1 is legacy.
sv (CLI)	latest	Replaces `create-svelte`. Use `npx sv` for all scaffolding.

Scaffolding a New Project

bash

# Current recommended command (npm create svelte@latest is DEPRECATED)
npx sv create my-app
cd my-app
npm install
npm run dev

sv create options

Flag	Purpose
`--template minimal\|demo\|library`	Project template
`--types ts\|jsdoc\|--no-types`	TypeScript config
`--add [add-ons...]`	Install add-ons (see below)
`--no-add-ons`	Skip interactive add-on prompt
`--install npm\|pnpm\|yarn\|bun\|deno`	Package manager

Available add-ons

drizzle, eslint, better-auth, tailwindcss, prettier, vitest, playwright, storybook, mdsvex, paraglide, sveltekit-adapter, mcp, devtools-json

TypeScript Setup

TypeScript is built-in. Select "ts" when prompted by npx sv create, or pass --types ts. No extra configuration needed.

•Use <script lang="ts"> in .svelte files
•Types are auto-generated on npm run dev or npx svelte-kit sync
•Required tsconfig options (set automatically): verbatimModuleSyntax: true, isolatedModules: true, noEmit: true
•.svelte.ts files for reactive modules (replaces .ts when you need runes)

Svelte 5 Runes (Core Reactivity)

Runes replace Svelte 4's implicit reactivity (let, $:) with explicit, portable primitives.

$state -- Reactive State

svelte

<script>
  let count = $state(0);           // primitive
  let todos = $state([]);           // deep reactive proxy (arrays/objects)
</script>
<button onclick={() => count++}>{count}</button>

•Deeply reactive: mutations to nested objects/arrays are tracked automatically
•Class fields: done = $state(false); inside class bodies

$state.raw -- Non-reactive (reassign-only)

let items = $state.raw([1, 2, 3]);
// items.push(4)  -- NO EFFECT (not reactive)
items = [...items, 4]; // works (reassignment)

Use for large collections you never mutate in place. Better performance.

$state.snapshot -- Static copy

let data = $state({ count: 0 });
console.log($state.snapshot(data)); // plain object, no proxy

Use when passing state to external libraries or structuredClone.

$derived -- Computed Values

let count = $state(0);
const doubled = $derived(count * 2);

$derived.by -- Complex derivations

const filtered = $derived.by(() => {
  return items.filter(item => item.active);
});

$derived(expr) is equivalent to $derived.by(() => expr).

$effect -- Side Effects

$effect(() => {
  // Runs on mount and whenever dependencies change
  // Dependencies are auto-tracked
  console.log(`count is ${count}`);

  // Optional cleanup (returned function runs before re-execution)
  return () => { /* cleanup */ };
});

$effect.pre -- Before DOM update

$effect.pre(() => {
  // Runs before DOM is updated. Rare use case.
});

Rule of thumb: $derived for values, $effect for actions/side effects.

$props -- Component Props

svelte

<script>
  // Replaces "export let"
  let { name, count = 0, class: klass, ...rest } = $props();
</script>

•Destructure with defaults, renaming, and rest props
•Props are NOT bindable by default (see $bindable)

$bindable -- Two-way Binding

svelte

<script>
  let { value = $bindable('default') } = $props();
</script>

Parent can then use bind:value={something}.

$inspect -- Debug

$inspect(count); // logs to console whenever count changes (dev only, stripped in prod)

Snippets (Replace Slots)

Slots are deprecated. Use snippets and {@render} instead.

Default content (children)

svelte

<!-- Parent -->
<Card>
  <p>This becomes the children snippet</p>
</Card>

<!-- Card.svelte -->
<script>
  let { children } = $props();
</script>
<div class="card">
  {@render children?.()}
</div>

Named snippets

svelte

<!-- Parent -->
<Card>
  {#snippet header()}
    <h2>Title</h2>
  {/snippet}
  {#snippet footer()}
    <p>Footer</p>
  {/snippet}
  <p>Default children content</p>
</Card>

<!-- Card.svelte -->
<script>
  let { header, footer, children } = $props();
</script>
<div class="card">
  {@render header?.()}
  {@render children?.()}
  {@render footer?.()}
</div>

Snippets with parameters

svelte

<!-- Parent -->
<List items={users}>
  {#snippet item(user)}
    <span>{user.name}</span>
  {/snippet}
</List>

<!-- List.svelte -->
<script>
  let { items, item } = $props();
</script>
{#each items as entry}
  {@render item(entry)}
{/each}

Event Handling Changes

DOM events: remove the colon

svelte

<!-- Svelte 4 -->
<button on:click={handler}>Click</button>
<input on:input={handler} />

<!-- Svelte 5 -->
<button onclick={handler}>Click</button>
<input oninput={handler} />

Component events: callback props replace createEventDispatcher

svelte

<!-- Svelte 4 child -->
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();
</script>
<button on:click={() => dispatch('submit', data)}>Submit</button>

<!-- Svelte 5 child -->
<script>
  let { onsubmit } = $props();
</script>
<button onclick={() => onsubmit(data)}>Submit</button>

Event modifiers are gone

Replace on:click|preventDefault with explicit calls:

svelte

<button onclick={(e) => { e.preventDefault(); handler(e); }}>Click</button>

Component Mounting (Programmatic)

// Svelte 4
const app = new App({ target: document.getElementById('app') });

// Svelte 5
import { mount, unmount } from 'svelte';
const app = mount(App, { target: document.getElementById('app') });
// later: unmount(app);

Cross-Module Reactive State (.svelte.ts files)

// counter.svelte.ts
let count = $state(0);

// Cannot export reassignable $state directly. Two patterns:

// Pattern 1: Export object (mutate properties)
export const counter = $state({ count: 0 });
export function increment() { counter.count++; }

// Pattern 2: Export getter functions
export function getCount() { return count; }
export function increment() { count++; }

SSR caveat: Module-scoped $state is a singleton shared across SSR requests. Always reset at render start. See "SSR Behavior for Module-Scoped State" below.

SSR Behavior for Module-Scoped State

What runs during SSR vs CSR

Context	SSR	CSR
Top-level `<script>` code	YES	YES
`$state` initialization	YES	YES
`$derived` / `$derived.by`	Compiled as IIFE (computed once at module init)	Reactive (recomputes on dependency change)
`$effect` / `$effect.pre`	NO	YES
`onMount`	NO	YES
Template expressions	YES	YES

The $derived SSR trap

$derived.by(() => ...) compiles to const x = (() => ...)() during SSR -- an immediately-invoked function that runs once at module initialization, not reactively. If module-scoped $state is set after the derived is initialized, the derived will NOT reflect the updated values.

Broken pattern:

// store.svelte.ts
let page = $state('');
const snapshot = $derived.by(() => ({ page })); // Frozen at init during SSR!
function setPage(p) { page = p; }

Working pattern:

// store.svelte.ts
let page = $state('');
function buildSnapshot() { return { page }; } // Fresh read every call
export const store = {
  get snapshot() { return buildSnapshot(); }, // Works in both SSR and CSR
  setPage(p) { page = p; },
};

During CSR, reading $state variables inside a getter called from a template still triggers Svelte 5's fine-grained reactivity tracking.

SSR baseline pattern for layouts/pages

To populate state during SSR, call setters at the top level of <script> blocks (not in $effect or onMount). Use $effect for client-side reactive updates:

svelte

<script>
  import { page } from '$app/stores';
  import { myStore } from '$lib/stores/my.svelte';

  let { data } = $props();

  // SSR + CSR: runs on every render
  myStore.setPage(data.title);

  // CSR only: reactive updates on navigation
  $effect(() => {
    myStore.setPage(data.title);
  });
</script>

Avoiding state_referenced_locally warnings

When you need $derived values for SSR baseline code, DON'T read the $derived at top level. Instead, compute the value directly from the source:

svelte

<!-- BAD: warns because $derived is read outside reactive context -->
<script>
  let isAdmin = $derived(data.user?.role === 'admin');
  if (isAdmin) { store.setAuth({...}); } // Warning!
</script>

<!-- GOOD: compute from source directly -->
<script>
  let isAdmin = $derived(data.user?.role === 'admin');
  // SSR baseline: use source data, not $derived
  if (data.user?.role === 'admin') { store.setAuth({...}); }
  // $effect for CSR reactivity uses $derived normally
  $effect(() => { if (isAdmin) { store.setAuth({...}); } });
</script>

Module-scoped $state and SSR safety

Module-scoped $state is shared across all SSR requests (it's a singleton). SvelteKit SSR is synchronous per-request, so call reset() at the top of the root layout to prevent cross-request pollution:

svelte

<!-- +layout.svelte (root) -->
<script>
  import { myStore } from '$lib/stores/my.svelte';
  myStore.reset(); // Clear state before each render
</script>

SvelteKit 2 Key Differences from SvelteKit 1

•error() and redirect() no longer need to be thrown -- just call them
•cookies.set/delete/serialize require explicit path parameter
•Import vitePreprocess from @sveltejs/vite-plugin-svelte (not from SvelteKit)
•resolvePath replaced by resolveRoute from $app/paths
•Paths are consistently relative or absolute based on paths.relative config
•Requires Node 18.13+

Migration Commands

bash

# Migrate Svelte 4 -> Svelte 5
npx sv migrate svelte-5

# Migrate SvelteKit 1 -> SvelteKit 2
npx sv migrate sveltekit-2

Project Structure (SvelteKit)

code

src/
  lib/           # Shared code, importable as $lib/
  routes/        # File-based routing
    +page.svelte        # Page component
    +page.ts            # Page load function (universal)
    +page.server.ts     # Server-only load / form actions
    +layout.svelte      # Layout component
    +layout.ts          # Layout load function
    +layout.server.ts   # Server-only layout load
    +error.svelte       # Error page
    +server.ts          # API endpoint (GET, POST, etc.)
  app.html       # HTML template
  app.d.ts       # Auto-generated types
svelte.config.js # SvelteKit configuration
vite.config.ts   # Vite configuration

Common Patterns

Form Actions (server-side form handling)

svelte

<!-- +page.svelte -->
<form method="POST" action="?/create">
  <input name="title" />
  <button>Create</button>
</form>

// +page.server.ts
import type { Actions } from './$types';

export const actions = {
  create: async ({ request }) => {
    const data = await request.formData();
    const title = data.get('title');
    // handle creation
  }
} satisfies Actions;

Load Functions

// +page.ts (runs on server AND client)
import type { PageLoad } from './$types';

export const load: PageLoad = async ({ fetch }) => {
  const res = await fetch('/api/data');
  return { items: await res.json() };
};

// +page.server.ts (runs ONLY on server)
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async ({ locals }) => {
  return { user: locals.user };
};

SvelteKit + Svelte 5 Quick Reference (February 2026)

Current Versions

Scaffolding a New Project

sv create options

Available add-ons

TypeScript Setup

Svelte 5 Runes (Core Reactivity)

$state -- Reactive State

$state.raw -- Non-reactive (reassign-only)

$state.snapshot -- Static copy

$derived -- Computed Values

$derived.by -- Complex derivations

$effect -- Side Effects

$effect.pre -- Before DOM update

$props -- Component Props

$bindable -- Two-way Binding

$inspect -- Debug

Snippets (Replace Slots)

Default content (children)

Named snippets

Snippets with parameters

Event Handling Changes

DOM events: remove the colon

Component events: callback props replace createEventDispatcher

Event modifiers are gone

Component Mounting (Programmatic)

Cross-Module Reactive State (.svelte.ts files)

SSR Behavior for Module-Scoped State

What runs during SSR vs CSR

The $derived SSR trap

SSR baseline pattern for layouts/pages

Avoiding state_referenced_locally warnings

Module-scoped $state and SSR safety

SvelteKit 2 Key Differences from SvelteKit 1

Migration Commands

Project Structure (SvelteKit)

Common Patterns

Form Actions (server-side form handling)

Load Functions

Sources