Svelte 5 Runes vs Svelte 4 Reactivity Explained

Svelte 5 introduces a fundamental shift in how reactivity is managed by moving from a purely compiler-driven model to an explicit, signal-based system powered by "runes." While Svelte 4 relied on implicit compiler transformations and syntax overloading (such as let declarations and $: labels) scoped strictly to .svelte files, Svelte 5 provides explicit primitives like $state, $derived, and $effect. This transition eliminates historical edge cases, standardizes reactivity across components and external JavaScript modules, and improves runtime performance through fine-grained updates.

The Svelte 4 Approach: Implicit Compiler Magic

In Svelte 4, reactivity was governed entirely by the compiler parsing standard JavaScript declarations within single-file components:

• Variable Declarations: Simple let count = 0 assignments became reactive state. Re-rendering was triggered exclusively through reassignment (count += 1).
• Reactive Statements: The JavaScript label syntax $: doubled = count * 2 was hijacked to declare derived values and manage side effects.
• Component-Bound Scope: Reactivity was tied strictly to the .svelte file markup. To share reactive state outside of components, developers had to switch paradigms and use Svelte stores (writable, derived, readable).

While intuitive for beginners, this approach created limitations. Mutating deep object properties often failed to trigger updates without explicit reassignments (e.g., arr = arr). Furthermore, the $: label combined derived values and side effects into a single overloaded syntax, making execution order and dependency tracking complex in large codebases.

Svelte 5's Runes: Explicit Signals

Svelte 5 replaces implicit compiler magic with "runes"—function-like language extensions that tell the compiler how to handle reactivity using a modern signal-based engine.

1. Defining State with $state

Instead of treating every top-level let as reactive, Svelte 5 uses $state:

let count = $state(0);
let user = $state({ name: 'Alice' });

Unlike Svelte 4, $state provides deep reactivity via JavaScript proxies. Mutating a nested property (e.g., user.name = 'Bob') automatically triggers a reactive update without requiring reassignments or immutable clones.

2. Computed Values with $derived

The ambiguous $: label is replaced with $derived:

let doubled = $derived(count * 2);

$derived makes pure computations explicit and separates them from side effects. Dependencies are tracked dynamically at runtime, ensuring computations only recalculate when their specific dependencies change.

3. Managing Side Effects with $effect

Side effects are decoupled from derived calculations using the $effect rune:

$effect(() => {
    console.log(`The count is now ${count}`);
    return () => {
        console.log('Cleanup logic runs before the next execution');
    };
});

$effect runs exclusively in the browser after the DOM has updated, providing built-in cleanup phases and preventing unexpected execution during server-side rendering.

4. Standardized Props with $props

Passing data between components in Svelte 4 relied on the export let pattern. Svelte 5 replaces this with $props:

let { title, count = 0 } = $props();

This syntax aligns directly with modern JavaScript destructuring, supports default values cleanly, and allows type definitions to map directly to standard TypeScript interfaces.

Universal Reactivity Outside Components

The most practical distinction between the two versions is universal reactivity. In Svelte 4, standard reactivity ceased to function once logic was moved out of a .svelte file, forcing the use of Svelte stores with manual subscriptions ($store).

In Svelte 5, runes function identically inside .svelte components and standard .svelte.js or .svelte.ts files. Developers can encapsulate complex, fine-grained reactive state and logic inside vanilla functions without changing APIs or managing store subscriptions.

Architectural Difference: Compile-Time vs. Runtime Signals

Under the hood, Svelte 4 relied on compile-time dirty-checking: the compiler identified which variables were modified, flagged components as "dirty," and scheduled component-level reconciliations.

Svelte 5 utilizes runtime signals. The compiler still optimizes the output, but the runtime tracks direct dependencies between state and the specific DOM nodes that display it. When a value changes, only the affected DOM node updates, eliminating unnecessary re-evaluations and significantly improving performance in data-heavy applications.