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Key blocks destroy and recreate their contents when the value of an expression changes. This is useful for forcing components to reinitialize or triggering transitions.

​
Basic Syntax

{#key expression}...{/key}

​
Reinitializing Components

When used around components, key blocks cause them to be reinstantiated and reinitialized: 
{#key value}
  <Component />
{/key}

​
Example: Resetting Component State

<script>
  import Form from './Form.svelte';
  let formKey = $state(0);
  
  function resetForm() {
    formKey++; // Increment to force recreation
  }
</script>

<button onclick={resetForm}>Reset Form</button>

{#key formKey}
  <Form />
{/key}

​
Triggering Transitions

Key blocks are useful for playing transitions whenever a value changes: 
{#key value}
  <div transition:fade>{value}</div>
{/key}

​
Example: Animated Counter

<script>
  import { fade, fly } from 'svelte/transition';
  let count = $state(0);
</script>

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

{#key count}
  <div transition:fly={{ y: -20 }}>
    <h1>{count}</h1>
  </div>
{/key}

​
Real-World Use Cases

Route Transitions

<script>
  import { page } from '$app/stores';
  import { fade } from 'svelte/transition';
</script>

{#key $page.url.pathname}
  <div transition:fade>
    <slot />
  </div>
{/key}

Data Refresh

<script>
  let userId = $state(1);
  let refreshKey = $state(0);
  
  function refresh() {
    refreshKey++;
  }
</script>

<button onclick={refresh}>Refresh Data</button>

{#key refreshKey}
  {#await fetchUser(userId)}
    <p>Loading...</p>
  {:then user}
    <UserProfile {user} />
  {/await}
{/key}

Media Player

<script>
  let currentTrack = $state(tracks[0]);
</script>

{#key currentTrack.id}
  <video 
    src={currentTrack.url} 
    autoplay
  >
    <track kind="captions" src={currentTrack.captions} />
  </video>
{/key}

Animated Messages

<script>
  import { fly } from 'svelte/transition';
  let message = $state('');
  let messages = $state([]);
  
  function sendMessage() {
    messages = [...messages, {
      id: Date.now(),
      text: message
    }];
    message = '';
  }
</script>

{#each messages as msg (msg.id)}
  {#key msg.id}
    <div transition:fly={{ x: 300 }}>
      {msg.text}
    </div>
  {/key}
{/each}

​
Resetting Form State

Key blocks are perfect for resetting forms without manually clearing each field: 
<script>
  let formKey = $state(0);
  let submitted = $state(false);
  
  function handleSubmit(event) {
    const formData = new FormData(event.target);
    // Process form...
    submitted = true;
    
    // Reset form by changing key
    formKey++;
  }
</script>

{#key formKey}
  <form onsubmit={handleSubmit}>
    <input name="name" placeholder="Name" />
    <input name="email" type="email" placeholder="Email" />
    <textarea name="message" placeholder="Message"></textarea>
    <button type="submit">Submit</button>
  </form>
{/key}

{#if submitted}
  <p class="success">Form submitted successfully!</p>
{/if}

<script>
  import { fade } from 'svelte/transition';
  import { quintOut } from 'svelte/easing';
  
  let images = $state([
    { id: 1, src: '/images/photo1.jpg', alt: 'Photo 1' },
    { id: 2, src: '/images/photo2.jpg', alt: 'Photo 2' },
    { id: 3, src: '/images/photo3.jpg', alt: 'Photo 3' }
  ]);
  
  let currentIndex = $state(0);
  let current = $derived(images[currentIndex]);
  
  function next() {
    currentIndex = (currentIndex + 1) % images.length;
  }
  
  function prev() {
    currentIndex = (currentIndex - 1 + images.length) % images.length;
  }
</script>

<div class="gallery">
  <button onclick={prev}>Previous</button>
  
  {#key current.id}
    <img 
      src={current.src} 
      alt={current.alt}
      transition:fade={{ duration: 300, easing: quintOut }}
    />
  {/key}
  
  <button onclick={next}>Next</button>
</div>

​
Dynamic Component Switching

<script>
  import TabA from './TabA.svelte';
  import TabB from './TabB.svelte';
  import TabC from './TabC.svelte';
  
  let activeTab = $state('a');
  
  const components = {
    a: TabA,
    b: TabB,
    c: TabC
  };
</script>

<div class="tabs">
  <button onclick={() => activeTab = 'a'}>Tab A</button>
  <button onclick={() => activeTab = 'b'}>Tab B</button>
  <button onclick={() => activeTab = 'c'}>Tab C</button>
</div>

{#key activeTab}
  <svelte:component this={components[activeTab]} />
{/key}

​
Performance Considerations

Key blocks completely destroy and recreate their contents. This means:
  • All component state is lost
  • DOM elements are removed and recreated
  • Lifecycle methods run again
Use key blocks intentionally, not as a default pattern.

​
When to Use Key Blocks

  • Force a component to reset to its initial state
  • Trigger enter/exit transitions on value changes
  • Ensure media elements reload with new sources
  • Reset complex third-party components

​
When NOT to Use Key Blocks

  • Simple value updates (use reactive statements instead)
  • List rendering (use {#each} with keys)
  • Conditional rendering (use {#if} blocks)
  • Minor UI updates

​
Combining with Other Blocks

<script>
  import { slide } from 'svelte/transition';
  let category = $state('all');
  let items = $state([]);
</script>

<select bind:value={category}>
  <option value="all">All Items</option>
  <option value="featured">Featured</option>
  <option value="new">New Arrivals</option>
</select>

{#key category}
  <div transition:slide>
    {#await fetchItems(category)}
      <p>Loading {category} items...</p>
    {:then items}
      {#each items as item (item.id)}
        <div class="item">{item.name}</div>
      {:else}
        <p>No items in this category</p>
      {/each}
    {/await}
  </div>
{/key}

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