Svelte 5 Runes

This skill covers the Svelte 5 runes system, which provides powerful primitives for reactive state management, derived values, and side effects in Svelte applications.

What Are Runes?

Runes are symbols that control the Svelte compiler and provide explicit reactive primitives. They replace the implicit reactivity of Svelte 4 with a more powerful and predictable system.

Key benefits:

• Explicit reactivity that works everywhere (not just component top-level)
• Better TypeScript support
• More predictable behavior
• Works in both components and regular JavaScript/TypeScript files

Reference: https://svelte.dev/docs/svelte/llms.txt

The $state Rune

The $state rune declares reactive state that automatically triggers UI updates when changed.

Basic Usage

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

function increment() {
  count += 1;
}
</script>

<button onclick={increment}>
  Clicks: {count}
</button>

State with Objects

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

function updateName(newName) {
  user.name = newName;
}

function incrementAge() {
  user.age += 1;
}
</script>

<input value={user.name} oninput={(e) => updateName(e.target.value)} />
<p>{user.name} is {user.age} years old</p>
<button onclick={incrementAge}>Birthday!</button>

State with Arrays

<script>
let items = $state([]);

function addItem(item) {
  items.push(item);
}

function removeItem(index) {
  items.splice(index, 1);
}

function clearItems() {
  items = [];
}
</script>

<button onclick={() => addItem('New Item')}>Add Item</button>
<button onclick={clearItems}>Clear All</button>

<ul>
  {#each items as item, i}
    <li>
      {item}
      <button onclick={() => removeItem(i)}>Remove</button>
    </li>
  {/each}
</ul>

Deep Reactivity

<script>
let app = $state({
  user: {
    profile: {
      name: 'Bob',
      settings: {
        theme: 'dark'
      }
    }
  }
});

function toggleTheme() {
  // Deep updates work automatically
  app.user.profile.settings.theme = 
    app.user.profile.settings.theme === 'dark' ? 'light' : 'dark';
}
</script>

<p>Current theme: {app.user.profile.settings.theme}</p>
<button onclick={toggleTheme}>Toggle Theme</button>

The $derived Rune

The $derived rune creates values that automatically update when their dependencies change.

Basic Derived Values

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

function increment() {
  count += 1;
}
</script>

<p>Count: {count}</p>
<p>Doubled: {doubled}</p>
<p>Quadrupled: {quadrupled}</p>
<button onclick={increment}>Increment</button>

Derived with Complex Logic

<script>
let items = $state([
  { name: 'Apple', price: 1.5, quantity: 3 },
  { name: 'Banana', price: 0.5, quantity: 6 },
  { name: 'Orange', price: 2.0, quantity: 2 }
]);

let total = $derived(
  items.reduce((sum, item) => sum + item.price * item.quantity, 0)
);

let averagePrice = $derived(
  items.length > 0 
    ? items.reduce((sum, item) => sum + item.price, 0) / items.length
    : 0
);

function updateQuantity(index, newQuantity) {
  items[index].quantity = newQuantity;
}
</script>

<p>Total: ${total.toFixed(2)}</p>
<p>Average Price: ${averagePrice.toFixed(2)}</p>

{#each items as item, i}
  <div>
    {item.name}: ${item.price} x {item.quantity}
    <input 
      type="number" 
      value={item.quantity}
      oninput={(e) => updateQuantity(i, +e.target.value)}
    />
  </div>
{/each}

Chaining Derived Values

<script>
let celsius = $state(0);
let fahrenheit = $derived((celsius * 9/5) + 32);
let kelvin = $derived(celsius + 273.15);
let temperatureDescription = $derived(
  celsius < 0 ? 'Freezing' :
  celsius < 10 ? 'Cold' :
  celsius < 20 ? 'Cool' :
  celsius < 30 ? 'Warm' : 'Hot'
);

function setCelsius(value) {
  celsius = value;
}
</script>

<input 
  type="range" 
  min="-20" 
  max="50" 
  value={celsius}
  oninput={(e) => setCelsius(+e.target.value)}
/>

<p>{celsius}°C = {fahrenheit.toFixed(1)}°F = {kelvin.toFixed(1)}K</p>
<p>Status: {temperatureDescription}</p>

Derived with Conditionals

<script>
let user = $state({ isLoggedIn: false, name: '', role: '' });

let greeting = $derived(
  user.isLoggedIn 
    ? `Welcome back, ${user.name}!`
    : 'Please log in'
);

let permissions = $derived(
  user.role === 'admin' 
    ? ['read', 'write', 'delete']
    : user.role === 'editor'
    ? ['read', 'write']
    : ['read']
);

function login(name, role) {
  user.isLoggedIn = true;
  user.name = name;
  user.role = role;
}
</script>

<p>{greeting}</p>
<p>Permissions: {permissions.join(', ')}</p>
<button onclick={() => login('Alice', 'admin')}>Login as Admin</button>

The $derived.by Rune

The $derived.by rune is used when the derivation logic is complex enough to warrant a function.

Complex Calculations

<script>
let numbers = $state([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);

let statistics = $derived.by(() => {
  const sum = numbers.reduce((a, b) => a + b, 0);
  const mean = sum / numbers.length;
  const sorted = [...numbers].sort((a, b) => a - b);
  const median = sorted[Math.floor(sorted.length / 2)];
  const max = Math.max(...numbers);
  const min = Math.min(...numbers);
  
  return { sum, mean, median, max, min };
});

function addNumber(num) {
  numbers.push(num);
}
</script>

<p>Sum: {statistics.sum}</p>
<p>Mean: {statistics.mean.toFixed(2)}</p>
<p>Median: {statistics.median}</p>
<p>Max: {statistics.max}</p>
<p>Min: {statistics.min}</p>

Filtering and Mapping

<script>
let tasks = $state([
  { id: 1, text: 'Buy groceries', completed: false, priority: 'high' },
  { id: 2, text: 'Walk dog', completed: true, priority: 'medium' },
  { id: 3, text: 'Write code', completed: false, priority: 'high' }
]);

let filter = $state('all');

let filteredTasks = $derived.by(() => {
  if (filter === 'completed') {
    return tasks.filter(t => t.completed);
  } else if (filter === 'active') {
    return tasks.filter(t => !t.completed);
  } else if (filter === 'high-priority') {
    return tasks.filter(t => t.priority === 'high');
  }
  return tasks;
});

let taskStats = $derived.by(() => {
  const total = tasks.length;
  const completed = tasks.filter(t => t.completed).length;
  const active = total - completed;
  const completionRate = total > 0 ? (completed / total * 100).toFixed(0) : 0;
  
  return { total, completed, active, completionRate };
});

function toggleTask(id) {
  const task = tasks.find(t => t.id === id);
  if (task) task.completed = !task.completed;
}
</script>

<div>
  <button onclick={() => filter = 'all'}>All</button>
  <button onclick={() => filter = 'active'}>Active</button>
  <button onclick={() => filter = 'completed'}>Completed</button>
  <button onclick={() => filter = 'high-priority'}>High Priority</button>
</div>

<p>Stats: {taskStats.completed}/{taskStats.total} completed ({taskStats.completionRate}%)</p>

{#each filteredTasks as task}
  <div>
    <input type="checkbox" checked={task.completed} onchange={() => toggleTask(task.id)} />
    {task.text} ({task.priority})
  </div>
{/each}

Async Derived Values

<script>
let userId = $state(1);

let userData = $derived.by(async () => {
  const response = await fetch(`https://api.example.com/users/${userId}`);
  return await response.json();
});
</script>

{#await userData}
  <p>Loading user data...</p>
{:then user}
  <p>Name: {user.name}</p>
  <p>Email: {user.email}</p>
{:catch error}
  <p>Error: {error.message}</p>
{/await}

The $effect Rune

The $effect rune runs side effects when its dependencies change.

Basic Side Effects

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

$effect(() => {
  console.log(`Count changed to: ${count}`);
});

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

function increment() {
  count += 1;
}
</script>

<button onclick={increment}>Count: {count}</button>

localStorage Synchronization

<script>
let theme = $state(localStorage.getItem('theme') || 'light');

$effect(() => {
  localStorage.setItem('theme', theme);
  document.body.className = theme;
});

function toggleTheme() {
  theme = theme === 'light' ? 'dark' : 'light';
}
</script>

<button onclick={toggleTheme}>Toggle Theme</button>
<p>Current theme: {theme}</p>

API Calls

<script>
let searchQuery = $state('');
let results = $state([]);
let isLoading = $state(false);

$effect(() => {
  if (searchQuery.length < 3) {
    results = [];
    return;
  }

  isLoading = true;
  
  const controller = new AbortController();
  
  fetch(`https://api.example.com/search?q=${searchQuery}`, {
    signal: controller.signal
  })
    .then(res => res.json())
    .then(data => {
      results = data;
      isLoading = false;
    })
    .catch(err => {
      if (err.name !== 'AbortError') {
        console.error(err);
        isLoading = false;
      }
    });
  
  return () => {
    controller.abort();
  };
});
</script>

<input 
  type="text" 
  value={searchQuery}
  oninput={(e) => searchQuery = e.target.value}
  placeholder="Search (min 3 chars)..."
/>

{#if isLoading}
  <p>Loading...</p>
{:else}
  <ul>
    {#each results as result}
      <li>{result.name}</li>
    {/each}
  </ul>
{/if}

Cleanup Functions

<script>
let isActive = $state(false);

$effect(() => {
  if (!isActive) return;
  
  const interval = setInterval(() => {
    console.log('Tick');
  }, 1000);
  
  return () => {
    clearInterval(interval);
    console.log('Cleanup: interval cleared');
  };
});

function toggle() {
  isActive = !isActive;
}
</script>

<button onclick={toggle}>
  {isActive ? 'Stop' : 'Start'} Timer
</button>

Migration from Svelte 4

Reactive Declarations

<!-- Svelte 4 -->
<script>
let count = 0;
$: doubled = count * 2;
$: console.log('count changed:', count);
</script>

<!-- Svelte 5 -->
<script>
let count = $state(0);
let doubled = $derived(count * 2);

$effect(() => {
  console.log('count changed:', count);
});
</script>

Stores to Runes

<!-- Svelte 4 with stores -->
<script>
import { writable, derived } from 'svelte/store';

const count = writable(0);
const doubled = derived(count, $count => $count * 2);
</script>

<p>Count: {$count}</p>
<p>Doubled: {$doubled}</p>
<button onclick={() => count.update(n => n + 1)}>Increment</button>

<!-- Svelte 5 with runes -->
<script>
let count = $state(0);
let doubled = $derived(count * 2);
</script>

<p>Count: {count}</p>
<p>Doubled: {doubled}</p>
<button onclick={() => count += 1}>Increment</button>

Props

<!-- Svelte 4 -->
<script>
export let name;
export let age = 0;
</script>

<!-- Svelte 5 -->
<script>
let { name, age = 0 } = $props();
</script>

Best Practices

Keep Derived Logic Pure

<script>
let items = $state([1, 2, 3]);

// GOOD: Pure derivation
let sum = $derived(items.reduce((a, b) => a + b, 0));

// AVOID: Side effects in derived
// let sum = $derived(() => {
//   console.log('calculating sum');  // Side effect!
//   return items.reduce((a, b) => a + b, 0);
// });

// Use $effect for side effects instead
$effect(() => {
  console.log('Sum changed:', sum);
});
</script>

Minimize Effect Dependencies

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

// GOOD: Specific effect
$effect(() => {
  console.log('Count changed:', count);
});

// AVOID: Unnecessary dependencies
// $effect(() => {
//   console.log('Count changed:', count);
//   // This effect reruns when name changes too
//   const userName = name;
// });
</script>

Use $derived.by for Complex Logic

<script>
let data = $state([...largeDataset]);

// GOOD: Complex logic in $derived.by
let processed = $derived.by(() => {
  return data
    .filter(item => item.active)
    .map(item => transform(item))
    .sort((a, b) => compare(a, b));
});

// AVOID: Inline complex logic in $derived
// let processed = $derived(data.filter(...).map(...).sort(...));
</script>

This skill provides a comprehensive foundation for using Svelte 5 runes effectively in your applications.