svelte-development

What this skill does

# Svelte Development Skill

This skill provides comprehensive guidance for building modern Svelte applications using reactivity runes (Svelte 5), components, stores, lifecycle hooks, transitions, and animations based on official Svelte documentation.

## When to Use This Skill

Use this skill when:
- Building high-performance web applications with minimal JavaScript overhead
- Creating single-page applications (SPAs) with reactive UI
- Developing interactive user interfaces with compile-time optimization
- Building embedded widgets and components with small bundle sizes
- Implementing real-time dashboards and data visualizations
- Creating progressive web apps (PWAs) with excellent performance
- Developing component libraries with native reactivity
- Building server-side rendered applications with SvelteKit
- Migrating from frameworks with virtual DOM to compiled approach
- Creating accessible and performant web applications

## Core Concepts

### Reactivity with Runes (Svelte 5)

Svelte 5 introduces runes, a new way to declare reactive state with better TypeScript support and clearer semantics.

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

  function increment() {
    count++;
  }

  function updateAge() {
    user.age++;
  }
</script>

<button on:click={increment}>
  Count: {count}
</button>

<button on:click={updateAge}>
  {user.name} is {user.age} years old
</button>
```

**$derived Rune:**
```javascript
<script>
  let count = $state(0);
  let doubled = $derived(count * 2);
  let quadrupled = $derived(doubled * 2);

  // Complex derived values
  let users = $state([
    { name: 'Alice', active: true },
    { name: 'Bob', active: false },
    { name: 'Charlie', active: true }
  ]);

  let activeUsers = $derived(users.filter(u => u.active));
  let activeCount = $derived(activeUsers.length);
</script>

<p>Count: {count}</p>
<p>Doubled: {doubled}</p>
<p>Quadrupled: {quadrupled}</p>
<p>Active users: {activeCount}</p>
```

**$effect Rune:**
```javascript
<script>
  let count = $state(0);
  let name = $state('Alice');

  // Effect runs when dependencies change
  $effect(() => {
    console.log(`Count is now ${count}`);
    document.title = `Count: ${count}`;
  });

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

    return () => {
      clearInterval(interval);
    };
  });

  // Conditional effects
  $effect(() => {
    if (count > 10) {
      console.log('Count exceeded 10!');
    }
  });
</script>
```

**$props Rune:**
```javascript
<script>
  // Type-safe props in Svelte 5
  let { name, age = 18, onClick } = $props();

  // With TypeScript
  interface Props {
    name: string;
    age?: number;
    onClick?: () => void;
  }

  let { name, age = 18, onClick }: Props = $props();
</script>

<div>
  <h2>{name}</h2>
  <p>Age: {age}</p>
  {#if onClick}
    <button on:click={onClick}>Click me</button>
  {/if}
</div>
```

### Components

Components are the building blocks of Svelte applications. Each component is a single file with script, markup, and styles.

**Basic Component Structure:**
```svelte
<script>
  // Component logic
  let name = $state('World');
  let count = $state(0);

  function handleClick() {
    count++;
  }
</script>

<!-- Component markup -->
<div class="container">
  <h1>Hello {name}!</h1>
  <p>Count: {count}</p>
  <button on:click={handleClick}>Increment</button>
</div>

<!-- Component styles (scoped by default) -->
<style>
  .container {
    padding: 1rem;
    border: 1px solid #ccc;
    border-radius: 8px;
  }

  h1 {
    color: #ff3e00;
    font-size: 2rem;
  }

  button {
    background: #ff3e00;
    color: white;
    border: none;
    padding: 0.5rem 1rem;
    border-radius: 4px;
    cursor: pointer;
  }

  button:hover {
    background: #ff5722;
  }
</style>
```

**Component Props:**
```svelte
<!-- Card.svelte -->
<script>
  let { title, description, imageUrl, onClick } = $props();
</script>

<div class="card" on:click={onClick}>
  {#if imageUrl}
    <img src={imageUrl} alt={title} />
  {/if}
  <h3>{title}</h3>
  <p>{description}</p>
</div>

<style>
  .card {
    border: 1px solid #ddd;
    border-radius: 8px;
    padding: 1rem;
    cursor: pointer;
    transition: transform 0.2s;
  }

  .card:hover {
    transform: translateY(-4px);
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.1);
  }

  img {
    width: 100%;
    border-radius: 4px;
  }
</style>
```

**Component Events:**
```svelte
<!-- Button.svelte -->
<script>
  import { createEventDispatcher } from 'svelte';

  let { variant = 'primary', disabled = false } = $props();

  const dispatch = createEventDispatcher();

  function handleClick() {
    dispatch('click', { timestamp: Date.now() });
  }
</script>

<button
  class="btn {variant}"
  {disabled}
  on:click={handleClick}
>
  <slot />
</button>

<style>
  .btn {
    padding: 0.5rem 1rem;
    border: none;
    border-radius: 4px;
    cursor: pointer;
  }

  .primary {
    background: #ff3e00;
    color: white;
  }

  .secondary {
    background: #676778;
    color: white;
  }
</style>

<!-- Usage -->
<script>
  import Button from './Button.svelte';

  function handleButtonClick(event) {
    console.log('Clicked at:', event.detail.timestamp);
  }
</script>

<Button on:click={handleButtonClick}>Click me</Button>
<Button variant="secondary" on:click={handleButtonClick}>Secondary</Button>
```

**Slots and Composition:**
```svelte
<!-- Modal.svelte -->
<script>
  let { isOpen = false, onClose } = $props();
</script>

{#if isOpen}
  <div class="modal-overlay" on:click={onClose}>
    <div class="modal-content" on:click|stopPropagation>
      <button class="close-btn" on:click={onClose}>×</button>

      <div class="modal-header">
        <slot name="header">
          <h2>Modal Title</h2>
        </slot>
      </div>

      <div class="modal-body">
        <slot>
          <p>Modal content goes here</p>
        </slot>
      </div>

      <div class="modal-footer">
        <slot name="footer">
          <button on:click={onClose}>Close</button>
        </slot>
      </div>
    </div>
  </div>
{/if}

<style>
  .modal-overlay {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.5);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 1000;
  }

  .modal-content {
    background: white;
    border-radius: 8px;
    padding: 2rem;
    max-width: 500px;
    width: 90%;
    position: relative;
  }

  .close-btn {
    position: absolute;
    top: 1rem;
    right: 1rem;
    background: none;
    border: none;
    font-size: 2rem;
    cursor: pointer;
  }
</style>

<!-- Usage -->
<script>
  import Modal from './Modal.svelte';
  let isModalOpen = $state(false);
</script>

<button on:click={() => isModalOpen = true}>Open Modal</button>

<Modal {isModalOpen} onClose={() => isModalOpen = false}>
  <svelte:fragment slot="header">
    <h2>Custom Title</h2>
  </svelte:fragment>

  <p>This is custom modal content.</p>

  <svelte:fragment slot="footer">
    <button on:click={() => isModalOpen = false}>Cancel</button>
    <button on:click={handleSave}>Save</button>
  </svelte:fragment>
</Modal>
```

### Stores

Stores are observable values that can be shared across components.

**Writable Store:**
```javascript
// stores.js
import { writable } from 'svelte/store';

export const count = writable(0);

export const user = writable({
  name: 'Guest',
  loggedIn: false
});

export const todos = writable([]);

// Custom store with methods
function createCounter() {
  const { subscribe, set, update } = writable(0);

  return {
    subscribe,
    increment: () => update(n => n + 1),
    decrement: () => update(n => n - 1),
    reset: () => set(0)
  };
}

export const counter = createCounter();
```

**Using Stores in Components:**
```svelte
<script>
  import { count, user, counter } from './stores.js';

  // Auto-subscription with $
  $: