Svelte 5 Syntax

Always use Svelte 5 runes. Never use Svelte 4 patterns.

Svelte 4 → Svelte 5

Stores → Runes

Quick Reference

<script>
  // Props (with defaults and rest)
  let { required, optional = 'default', ...rest } = $props();

  // Two-way bindable prop
  let { value = $bindable() } = $props();

  // Reactive state
  let count = $state(0);
  let items = $state([]);      // arrays are deeply reactive
  let user = $state({ name: '' }); // objects too

  // Derived values
  let doubled = $derived(count * 2);
  let complex = $derived.by(() => {
    // multi-line logic here
    return expensiveCalc(count);
  });

  // Side effects
  $effect(() => {
    console.log(count);
    return () => cleanup(); // optional cleanup
  });
</script>

<!-- Events: native names, no colon -->
<button onclick={() => count++}>Click</button>
<input oninput={e => value = e.target.value} />

<!-- Render snippets (replaces slots) -->
{@render children?.()}

Snippets (Replace Slots)

<!-- Parent passes snippets -->
<Dialog>
  {#snippet header()}
    <h1>Title</h1>
  {/snippet}

  {#snippet footer(close)}
    <button onclick={close}>Done</button>
  {/snippet}
</Dialog>

<!-- Child renders them -->
<script>
  let { header, footer, children } = $props();
</script>
{@render header?.()}
{@render children?.()}
{@render footer?.(() => open = false)}

References

Load these when needed:

• references/typescript.md — Typing props, state, derived, snippets, events, context
• references/patterns.md — Context API, controlled inputs, forwarding props, async data, debouncing
• references/gotchas.md — Reactivity edge cases, effect pitfalls, binding quirks