Svelte 5 Syntax
Always use Svelte 5 runes. Never use Svelte 4 patterns.
Svelte 4 → Svelte 5
| Svelte 4 ❌ | Svelte 5 ✅ |
|---|---|
export let foo | let { foo } = $props() |
export let foo = 'default' | let { foo = 'default' } = $props() |
$: doubled = x * 2 | let doubled = $derived(x * 2) |
$: { sideEffect() } | $effect(() => { sideEffect() }) |
on:click={handler} | onclick={handler} |
on:input={handler} | oninput={handler} |
on:click|preventDefault={h} | onclick={e => { e.preventDefault(); h(e) }} |
<slot /> | {@render children()} |
<slot name="x" /> | {@render x?.()} |
$$props | Use $props() with rest: let { ...rest } = $props() |
$$restProps | let { known, ...rest } = $props() |
createEventDispatcher() | Pass callback props: let { onchange } = $props() |
Stores → Runes
| Svelte 4 ❌ | Svelte 5 ✅ |
|---|---|
import { writable } from 'svelte/store' | Remove import |
const count = writable(0) | let count = $state(0) |
$count (auto-subscribe) | count (direct access) |
count.set(1) | count = 1 |
count.update(n => n + 1) | count += 1 |
Quick Reference
svelte
<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)
svelte
<!-- 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