Svelte5 Development

This skill provides guidance for Svelte 5 and SvelteKit development, covering runes, component patterns, routing, and common pitfalls.

Svelte 5 Runes - Core Reactivity

$state - Reactive State

Creates reactive state that updates the UI when changed.

<script>
	let count = $state(0);
	let user = $state({ name: 'Alice', age: 30 });
</script>

<button => count++}>Clicks: {count}</button>
<button => user.age++}>Age: {user.age}</button>

Deep Reactivity: Objects and arrays become deeply reactive proxies. Mutations trigger updates:

let todos = $state([{ done: false, text: 'learn svelte' }]);
todos[0].done = true; // triggers update
todos.push({ done: false, text: 'build app' }); // triggers update

Classes: Use $state in class fields:

class Todo {
	done = $state(false);
	constructor(text) {
		this.text = $state(text);
	}
	reset = () => {
		this.text = '';
		this.done = false;
	}
}

Important: When you destructure reactive state, references are NOT reactive:

let { done, text } = todos[0];
todos[0].done = true; // `done` variable won't update

$state.raw: Use for non-reactive objects (performance optimization):

let data = $state.raw({ large: 'dataset' });
data.large = 'new value'; // no effect, must reassign entire object
data = { large: 'new value' }; // this works

$derived - Computed Values

Creates values that automatically update when dependencies change.

<script>
	let count = $state(0);
	let doubled = $derived(count * 2);
	let tripled = $derived(count * 3);
</script>

<p>{count} × 2 = {doubled}</p>
<p>{count} × 3 = {tripled}</p>

For complex logic, use $derived.by:

let numbers = $state([1, 2, 3]);
let total = $derived.by(() => {
	let sum = 0;
	for (const n of numbers) sum += n;
	return sum;
});

Critical Rule: NEVER update state inside $derived - it should be side-effect free.

Overriding deriveds (Svelte 5.25+): Useful for optimistic UI:

let likes = $derived(post.likes);

async function onclick() {
	likes += 1; // optimistic update
	try {
		await likePost();
	} catch {
		likes -= 1; // rollback on error
	}
}

$effect - Side Effects

Runs when state changes. Use for DOM manipulation, third-party libraries, analytics.

<script>
	let size = $state(50);
	let canvas;

	$effect(() => {
		const ctx = canvas.getContext('2d');
		ctx.clearRect(0, 0, canvas.width, canvas.height);
		// reruns when `size` changes
		ctx.fillRect(0, 0, size, size);
	});
</script>

<canvas bind:this={canvas} width="100" height="100"></canvas>

Lifecycle: Effects run after component mounts and after state changes (in microtask).

Teardown functions: Return a function to clean up:

$effect(() => {
	const interval = setInterval(() => count++, 1000);
	return () => clearInterval(interval); // cleanup
});

Dependencies: Automatically tracks any $state/$derived read synchronously. Async reads (after await) are NOT tracked:

$effect(() => {
	context.fillStyle = color; // tracked
	setTimeout(() => {
		context.fillRect(0, 0, size, size); // size NOT tracked!
	}, 0);
});

Conditional dependencies: Only depends on values read in the last run:

$effect(() => {
	if (condition) {
		confetti({ colors: [color] }); // only depends on color if condition is true
	}
});

$effect.pre: Runs BEFORE DOM updates (rare, for things like autoscroll).

CRITICAL - When NOT to use $effect:

• ❌ Don't synchronize state (use $derived instead)
• ❌ Don't use for computed values
• ✅ DO use for: canvas drawing, third-party libs, analytics, intervals, DOM manipulation

<!-- ❌ BAD - Don't do this -->
<script>
	let count = $state(0);
	let doubled = $state();
	$effect(() => {
		doubled = count * 2; // WRONG! Use $derived
	});
</script>

<!-- ✅ GOOD - Do this -->
<script>
	let count = $state(0);
	let doubled = $derived(count * 2);
</script>

$props - Component Props

Receives data from parent components.

<!-- Parent.svelte -->
<Child message="hello" count={42} />

<!-- Child.svelte -->
<script>
	let { message, count = 0 } = $props(); // destructuring with defaults
</script>

<p>{message} - {count}</p>

Renaming props (for reserved words or invalid identifiers):

let { super: trouper = 'default' } = $props();

Rest props:

let { a, b, ...others } = $props();

Type safety (TypeScript):

<script lang="ts">
	let { message }: { message: string } = $props();
	// or
	interface Props {
		message: string;
		count?: number;
	}
	let { message, count = 0 }: Props = $props();
</script>

Important: Props update reactively, but you should NOT mutate them (unless $bindable).

$bindable - Two-Way Binding

Allows child components to update parent state.

<!-- FancyInput.svelte -->
<script>
	let { value = $bindable(), ...props } = $props();
</script>

<input bind:value {...props} />

<!-- Parent.svelte -->
<script>
	import FancyInput from './FancyInput.svelte';
	let message = $state('hello');
</script>

<FancyInput bind:value={message} />
<p>{message}</p>

Fallback values:

let { value = $bindable('default value') } = $props();

Use sparingly: Most components should use callback props instead of $bindable.

Common Patterns and Pitfalls

Effect vs Derived

<script>
	let a = $state(1);
	let b = $state(2);

	// ✅ GOOD - Use $derived for computed values
	let sum = $derived(a + b);

	// ❌ BAD - Don't use $effect for this
	let sum2 = $state(0);
	$effect(() => {
		sum2 = a + b; // WRONG!
	});

	// ✅ GOOD - Use $effect for side effects only
	$effect(() => {
		console.log('Sum changed:', sum);
		analytics.track('calculation', { sum });
	});
</script>

Navigation with Remote Functions

When using SvelteKit's remoteFunctions feature, standard <a href> links don't work for client-side navigation.

<script>
	import { goto } from '$app/navigation';
</script>

<!-- ❌ DON'T use regular links with remote functions -->
<a href="/songs/{id}/edit">{title}</a>

<!-- ✅ DO use goto() -->
<button => goto(`/songs/${id}/edit`)}>{title}</button>

Style buttons as links:

.link-button {
	background: none;
	border: none;
	padding: 0;
	color: inherit;
	cursor: pointer;
	text-decoration: underline;
}

Async Operations and Form Initialization

<script>
	let song = $state({ current: null });
	let title = $state('');
	let initialized = $state(false);

	// ✅ GOOD - Use $effect for async initialization
	$effect(() => {
		if (song.current && !initialized) {
			title = song.current.title || '';
			initialized = true;
		}
	});
</script>

SvelteKit Routing

File Structure

• +page.svelte - Page component
• +page.js - Universal load (runs server + client)
• +page.server.js - Server-only load
• +layout.svelte - Layout component (wraps pages)
• +layout.js / +layout.server.js - Layout load functions
• +error.svelte - Error page
• +server.js - API endpoints

Dynamic Routes

• [slug] - Single parameter
• [...rest] - Rest parameter (catches multiple segments)
• [[optional]] - Optional parameter

Example: src/routes/blog/[slug]/+page.svelte matches /blog/hello-world

Loading Data

Page Load (+page.js or +page.server.js):

/** @type {import('./$types').PageLoad} */
export function load({ params, url, fetch }) {
	return {
		post: {
			title: `Post ${params.slug}`,
			content: 'Content here'
		}
	};
}

Layout Load (+layout.server.js):

/** @type {import('./$types').LayoutServerLoad} */
export async function load() {
	return {
		sections: [
			{ slug: 'profile', title: 'Profile' },
			{ slug: 'settings', title: 'Settings' }
		]
	};
}

Accessing Data in Components:

<script>
	/** @type {import('./$types').PageProps} */
	let { data } = $props();
</script>

<h1>{data.post.title}</h1>

Parent Data Access:

export async function load({ parent }) {
	const { user } = await parent();
	return { username: user.name };
}

Universal vs Server Load

Use +page.js (Universal) when:

• Fetching from public APIs
• No private credentials needed
• Returning custom classes or constructors
• Running same logic server + client

Use +page.server.js (Server) when:

• Accessing database directly
• Using private environment variables
• Reading from filesystem
• Need to serialize data for client

Server load returns must be serializable (JSON + Date, Map, Set, RegExp, BigInt).

Form Actions

// +page.server.js
export const actions = {
	default: async ({ request }) => {
		const data = await request.formData();
		const email = data.get('email');
		// process form...
		return { success: true };
	}
};

<!-- +page.svelte -->
<script>
	/** @type {import('./$types').PageProps} */
	let { form } = $props();
</script>

{#if form?.success}
	<p>Success!</p>
{/if}

<form method="POST">
	<input name="email" type="email" />
	<button>Submit</button>
</form>

Real-time Updates (Server-Sent Events)

Server (+server.js):

export function GET() {
	const stream = new ReadableStream({
		start(controller) {
			const encoder = new TextEncoder();
			const send = (data) => {
				controller.enqueue(encoder.encode(`data: ${JSON.stringify(data)}\n\n`));
			};
			// Send updates...
			setInterval(() => send({ time: Date.now() }), 1000);
		}
	});

	return new Response(stream, {
		headers: {
			'Content-Type': 'text/event-stream',
			'Cache-Control': 'no-cache'
		}
	});
}

Client:

<script>
	let data = $state({ time: 0 });

	$effect(() => {
		const source = new EventSource('/updates');
		source.onmessage = (e) => {
			data = JSON.parse(e.data);
		};
		return () => source.close();
	});
</script>

Environment Variables

Public (exposed to client):

import { PUBLIC_STATION_NAME } from '$env/static/public';

Private (server-only):

import { DATABASE_URL } from '$env/static/private';

Component Patterns

Snippets and Render

Reusable markup patterns:

<script>
	let items = $state(['a', 'b', 'c']);
</script>

{#snippet listItem(item)}
	<li>{item}</li>
{/snippet}

<ul>
	{#each items as item}
		{@render listItem(item)}
	{/each}
</ul>

Conditional Rendering

{#if condition}
	<p>True</p>
{:else if otherCondition}
	<p>Other</p>
{:else}
	<p>False</p>
{/if}

Lists

{#each items as item, index (item.id)}
	<div>{index}: {item.name}</div>
{/each}

Always use keys (the (item.id) part) for dynamic lists!

Async/Await (Svelte 5.36+)

Requires experimental.async: true in config:

<script>
	async function fetchData() {
		const res = await fetch('/api/data');
		return res.json();
	}
</script>

<p>{await fetchData()}</p>

<!-- Or with #await -->
{#await fetchData()}
	Loading...
{:then data}
	<p>{data.message}</p>
{:catch error}
	<p>Error: {error.message}</p>
{/await}

Best Practices

1. Use $derived, not $effect, for computed values
2. Don't mutate props (use callbacks or $bindable)
3. Use $effect.pre only when necessary (before DOM updates)
4. Always provide keys in {#each} blocks for dynamic lists
5. Prefer server load functions for sensitive data
6. Use goto() for navigation when remote functions are enabled
7. Type your components with PageProps, LayoutProps, etc.

When to Consult External Resources

This skill covers the most common Svelte 5 and SvelteKit patterns. For advanced topics, refer to:

• SvelteKit adapters and deployment
• Advanced routing (route groups, breaking layouts)
• Hooks (handle, handleFetch, handleError)
• Service workers and offline support
• Custom Svelte stores (when not using runes)
• Migration from Svelte 4
• Testing strategies
• TypeScript advanced types

Remember: Start with the fundamentals in this skill, then explore advanced topics as needed.