Svelte 5 Runes - Reactivity Innovation

Svelte 5 Overview

Svelte 5 introduces a new reactivity system called “Runes.” It achieves more explicit and predictable reactivity with improved TypeScript compatibility.

What are Runes

Runes are special functions that start with $. The compiler recognizes these and transforms them into reactive code.

$state - Reactive State

  Count: {count}

  Count: {count}

Object State

{user.name} is {user.age} years old
Happy Birthday!

$derived - Derived Values

Define values computed from other state.

Area: {area}
Perimeter: {perimeter}

Complex Derivations

$effect - Side Effects

Execute side effects in response to state changes.

DOM Operations

$props - Component Properties

{name} is {age} years old
 onUpdate(age + 1)}>
  Increment

 console.log(newAge)} />

$bindable - Two-Way Binding

Value: {text}

Event Handling Changes

Click



Click

TypeScript Support

Migration Guide

# Auto-migration tool
npx sv migrate svelte-5

Gradual Migration

Svelte 5 maintains compatibility with legacy syntax, allowing gradual migration.

Performance

Benchmark (1000 element list update):
- Svelte 4: 12ms
- Svelte 5: 5ms (58% faster)

Bundle Size:
- Svelte 4 runtime: 18KB
- Svelte 5 runtime: 22KB (slightly increased due to new features)

Summary

Svelte 5’s Runes provide more explicit and predictable reactivity. With $state, $derived, and $effect, state management becomes intuitive, and TypeScript integration is improved.