Svelte Runes Reactivity: A New Mental Model
Svelte runes reactivity replaces Svelte’s compiler-magic reactivity with an explicit signal-based system using special $-prefixed functions called runes. Therefore, reactivity becomes predictable, composable, and works outside .svelte files in plain JavaScript modules. As a result, Svelte 5 eliminates the implicit reactivity gotchas that confused developers in previous versions.
Understanding $state and $derived
The $state rune creates reactive values that trigger UI updates when modified, similar to signals in other frameworks. Moreover, $derived computes values that automatically update when their dependencies change, replacing the $: label syntax. Consequently, the reactivity contract becomes explicit — you always know which values are reactive.
Deep reactivity tracks nested object and array mutations automatically without requiring immutable update patterns. Furthermore, $state.raw opts out of deep reactivity for large data structures where performance matters.
Svelte Runes Reactivity in Practice
Effects run automatically when reactive dependencies change, replacing beforeUpdate and afterUpdate lifecycle hooks. Additionally, $effect.pre runs before DOM updates while $effect runs after, covering all lifecycle timing needs. For example, a search component can debounce API calls reactively without manual subscription management.
// Svelte 5 runes-based component
<script>
let count = $state(0);
let doubled = $derived(count * 2);
let history = $state([]);
// Effect runs when count changes
$effect(() => {
history = [...history, count];
console.log(`Count changed to ${count}, doubled: ${doubled}`);
});
// Extracted reactive logic (works in .svelte.js files too)
function createTimer(interval) {
let elapsed = $state(0);
let running = $state(false);
$effect(() => {
if (!running) return;
const id = setInterval(() => elapsed++, interval);
return () => clearInterval(id); // cleanup
});
return {
get elapsed() { return elapsed; },
get running() { return running; },
start: () => running = true,
stop: () => running = false,
reset: () => elapsed = 0,
};
}
const timer = createTimer(1000);
</script>
<button onclick={() => count++}>Count: {count}</button>
<p>Doubled: {doubled}</p>
<p>Timer: {timer.elapsed}s</p>Reactive classes use $state in class fields for object-oriented patterns with automatic reactivity. Therefore, complex state management integrates naturally with TypeScript class-based architectures.
Migration from Svelte 4
The svelte-migrate tool automates most transformations from Svelte 4 reactive declarations to runes. However, some patterns like reactive assignments to exported props require manual review. In contrast to Svelte 4’s implicit reactivity, runes require explicit opt-in but eliminate surprising behavior.
Performance Benefits
Fine-grained reactivity updates only the specific DOM nodes affected by state changes rather than re-rendering entire component trees. Additionally, Svelte 5’s compiler generates more efficient update code that reduces bundle size compared to Svelte 4. Specifically, benchmarks show 20-40% faster updates for complex interactive applications.
Related Reading:
Further Resources:
In conclusion, Svelte runes reactivity provides a predictable and composable reactivity system that scales from simple components to complex applications. Therefore, migrate to Svelte 5 to benefit from explicit reactivity, better performance, and improved code reuse.