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+--- Thread: Svelte 5 Runes Deep Dive: Understanding the New Reactivity System (/svelte-5-runes-deep-dive-understanding-the-new-reactivity-system)
Svelte 5 Runes Deep Dive: Understanding the New Reactivity System - Admin - 03-25-2026
Svelte 5 introduced Runes, a completely new reactivity system that replaces the old $: reactive declarations. If you've been working with Svelte 3 or 4, this is a major paradigm shift. Let's break down everything you need to know.
What Are Runes?
Runes are special symbols that start with $ and tell the Svelte compiler how to handle reactivity. They replace the implicit reactivity of Svelte 3/4 with explicit, fine-grained reactivity primitives.
The Core Runes:
1. $state - Declares reactive state
let count = $state(0);
This replaces the old "let count = 0" that was implicitly reactive. Now reactivity is opt-in and explicit.
2. $derived - Computed values
let doubled = $derived(count * 2);
This replaces the old "$: doubled = count * 2" syntax. It automatically tracks dependencies and recalculates when they change.
3. $effect - Side effects
$effect(() => { console.log('Count changed:', count); });
Replaces "$: { console.log(count) }" blocks. Effects run after the DOM updates and automatically track their dependencies.
4. $props - Component props
let { name, age = 25 } = $props();
Replaces "export let name" declarations. Provides destructuring with defaults in a cleaner syntax.
5. $bindable - Two-way binding props
let { value = $bindable('') } = $props();
Used when a prop needs to support bind: directive from parent components.
Migration Tips:
- Use the official migration tool: npx sv migrate svelte-5
- Start with leaf components (components with no children) first
- Test thoroughly - reactivity behavior can differ in edge cases
- $state objects are deeply reactive by default, use $state.raw() for shallow reactivity
- $effect has cleanup support via returning a function
Performance Benefits:
- Fine-grained reactivity means fewer unnecessary re-renders
- Better dead code elimination
- Smaller bundle sizes due to optimized compilation
- Improved TypeScript integration with proper type inference
Common Mistakes:
- Destructuring $state objects breaks reactivity (use $state.snapshot() instead)
- Using $effect for derived values instead of $derived
- Forgetting that $state creates a proxy, not a plain object
What has your experience been with Svelte 5 Runes? Share your thoughts!