Svelte5 Development

Reactive State ($state)

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

<button onclick={() => count++}>
  Clicked {count} times
</button>

<button onclick={() => user.age++}>
  {user.name} is {user.age}
</button>

Deep reactivity: Objects and arrays update automatically on mutation.

Computed Values ($derived)

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

  // For complex computations
  let summary = $derived.by(() => {
    const total = items.length;
    const done = items.filter(i => i.done).length;
    return `${done}/${total}`;
  });
</script>

Side Effects ($effect)

<script>
  let count = $state(0);

  $effect(() => {
    console.log(`Count is ${count}`);
    document.title = `Count: ${count}`;

    // Cleanup function
    return () => {
      console.log('Cleanup');
    };
  });
</script>

Component Props ($props)

<!-- Button.svelte -->
<script>
  let { label, disabled = false, onclick } = $props();
</script>

<button {disabled} {onclick}>{label}</button>

TypeScript Props

<script lang="ts">
  interface Props {
    label: string;
    disabled?: boolean;
    onclick?: () => void;
  }

  let { label, disabled = false, onclick }: Props = $props();
</script>

SvelteKit File Conventions

Data Loading

// src/routes/posts/+page.server.js
export async function load({ fetch }) {
  const response = await fetch('/api/posts');
  return { posts: await response.json() };
}

<!-- src/routes/posts/+page.svelte -->
<script>
  let { data } = $props();
</script>

{#each data.posts as post}
  <article>
    <h2>{post.title}</h2>
  </article>
{/each}

Form Actions

// src/routes/login/+page.server.js
import { fail, redirect } from '@sveltejs/kit';

export const actions = {
  default: async ({ request, cookies }) => {
    const data = await request.formData();
    const email = data.get('email');

    if (!email) {
      return fail(400, { missing: true });
    }

    cookies.set('session', token, { path: '/' });
    throw redirect(303, '/dashboard');
  }
};

<!-- +page.svelte -->
<script>
  import { enhance } from '$app/forms';
  let { form } = $props();
</script>

<form method="POST" use:enhance>
  <input name="email" value={form?.email ?? ''} />
  {#if form?.missing}
    <p class="error">Email required</p>
  {/if}
  <button>Submit</button>
</form>

API Routes

// src/routes/api/posts/+server.js
import { json } from '@sveltejs/kit';

export async function GET({ url }) {
  const posts = await getPosts();
  return json(posts);
}

export async function POST({ request }) {
  const data = await request.json();
  const post = await createPost(data);
  return json(post, { status: 201 });
}

Common Pitfalls

Destructuring Breaks Reactivity

// ❌ Bad
let { count } = $state({ count: 0 });

// ✅ Good
let state = $state({ count: 0 });
state.count++;

Missing Keys in Each

<!-- ❌ Bad -->
{#each items as item}

<!-- ✅ Good -->
{#each items as item (item.id)}

Use Progressive Enhancement

<script>
  import { enhance } from '$app/forms';
</script>

<form method="POST" use:enhance>

Project Structure

src/
├── lib/
│   ├── components/
│   └── server/
├── routes/
│   ├── +layout.svelte
│   ├── +page.svelte
│   └── api/
├── app.html
└── hooks.server.js

Related Resources

See AgentUsage/svelte5_guide.md for complete documentation including:

• Advanced rune patterns
• Snippets for reusable markup
• Performance optimization
• TypeScript integration