Svelte 5 Runes: Complete Migration Guide

Svelte 5 arrives with a bold new feature called Runes, a declarative syntax that replaces the classic $: reactive statements and many store utilities. If you’ve built apps with Svelte 3 or 4, the shift feels like moving from a toolbox to a Swiss‑army knife – more powerful, but you need to know how to wield it. This guide walks you through every step of migrating an existing Svelte project to the Rune‑centric paradigm, complete with real‑world examples and pro tips to keep your bundle size lean.

Understanding Runes: The What and Why

Runes are essentially functions that return a reactive value. Unlike $: blocks, which run imperatively whenever a dependency changes, a Rune declares its dependencies up front, letting the compiler generate optimal update code. This results in fewer runtime checks, better tree‑shaking, and clearer intent for future maintainers.

In practice, a Rune looks like a regular JavaScript function that receives a run helper. Inside, you call run() with any reactive source (props, stores, or derived values). The function’s return value becomes the reactive output.

Key Differences from $: Statements

• Explicit dependency declaration via run() instead of implicit tracking.
• Return‑based reactivity – the Rune’s result updates automatically.
• Composable – you can nest Runes or combine them with regular functions.

Pro tip: Start by converting simple $: blocks to Runes; the pattern scales naturally to more complex logic.

Preparing Your Project for Migration

Before you touch a single line of component code, update your development environment. Svelte 5 requires svelte@5 and the latest vite or webpack plugin. Run the following commands:

npm install svelte@latest svelte-preprocess@latest
npm install -D @sveltejs/vite-plugin-svelte@latest

Next, enable the experimental Rune compiler flag in svelte.config.js. This ensures the compiler understands the new syntax while still allowing a gradual migration.

import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';

export default {
  preprocess: vitePreprocess(),
  compilerOptions: {
    // Turn on Rune support; remove after full migration
    runes: true
  }
};

Commit these changes to a new Git branch (e.g., migration/runes) so you can safely experiment without breaking the mainline.

Step 1: Converting Simple Reactive Assignments

Consider a classic $: block that derives a formatted date from a prop:

<script>
  export let timestamp;
  $: formatted = new Date(timestamp).toLocaleDateString();
</script>

<p>{formatted}</p>

With Runes, you replace the $: line with a function that returns the derived value:

<script>
  import { rune } from 'svelte/runes';
  export let timestamp;

  const formatted = rune(($run) => {
    $run(timestamp); // declare dependency
    return new Date(timestamp).toLocaleDateString();
  });
</script>

<p>{formatted}</p>

The component now has a clear, single source of truth: the formatted Rune. The compiler knows it only depends on timestamp, so it can skip updates when unrelated state changes.

Step 2: Migrating Reactive Statements with Multiple Dependencies

When a $: block references several variables, the implicit tracking can become opaque. Runes make each dependency explicit, improving readability and debugging.

<script>
  export let items = [];
  let filter = '';

  $: visible = items.filter(i => i.name.includes(filter));
</script>

<ul>
  {#each visible as item}
    <li>{item.name}</li>
  {/each}
</ul>

Converted to Runes:

<script>
  import { rune } from 'svelte/runes';
  export let items = [];
  let filter = '';

  const visible = rune(($run) => {
    $run(items);
    $run(filter);
    return items.filter(i => i.name.includes(filter));
  });
</script>

<ul>
  {#each $visible as item}
    <li>{item.name}</li>
  {/each}
</ul>

Notice the $visible usage in the markup. The dollar sign now signals “unwrap the Rune’s current value”, mirroring the old store syntax but with far less boilerplate.

Handling Conditional Logic Inside Runes

• Use plain if statements – the Rune will re‑run whenever any $run dependency changes.
• Avoid side effects inside Runes; keep them pure for maximum compiler optimizations.

Step 3: Replacing Store Subscriptions

Svelte stores (writable, readable, derived) have long used the $store auto‑subscription syntax. Runes unify this pattern: you can pass a store directly to $run, and the Rune will react to its updates.

Original store usage:

<script>
  import { writable } from 'svelte/store';
  const count = writable(0);

  function increment() {
    count.update(n => n + 1);
  }
</script>

<button on:click={increment}>Clicked {$count} times</button>

Rune‑based version:

<script>
  import { writable } from 'svelte/store';
  import { rune } from 'svelte/runes';
  const count = writable(0);

  const displayed = rune(($run) => {
    $run(count); // auto‑subscribe
    return $run(count);
  });

  function increment() {
    count.update(n => n + 1);
  }
</script>

<button on:click={increment}>Clicked {$displayed} times</button>

The displayed Rune simply mirrors the store, but you can now compose additional logic without creating separate derived stores.

Step 4: Event Handlers and Runes

Event handlers often combine local state with external data. With Runes, you can embed the handler logic inside a Rune, keeping the component’s script tidy.

Before migration:

<script>
  let query = '';
  function search() {
    fetch(`/api/search?q=${query}`)
      .then(r => r.json())
      .then(results => console.log(results));
  }
</script>

<input bind:value={query} placeholder="Search…" />
<button on:click={search}>Go</button>

After migration:

<script>
  import { rune } from 'svelte/runes';
  let query = '';

  const search = rune(($run) => {
    $run(query); // re‑create when query changes
    return async () => {
      const res = await fetch(`/api/search?q=${query}`);
      const results = await res.json();
      console.log(results);
    };
  });
</script>

<input bind:value={query} placeholder="Search…" />
<button on:click={$search}>Go</button>

Here $search is a callable Rune that always sees the latest query value, eliminating stale‑closure bugs common in older code.

Pro tip: When a handler needs only a subset of component state, pass those values into the Rune as arguments instead of relying on global $run tracking.

Step 5: Component APIs – Props, Slots, and Bindings

Component boundaries remain unchanged, but the way you expose reactive values does. Instead of exporting a $: variable, you export a Rune directly.

<script>
  import { rune } from 'svelte/runes';
  export let base = 10;

  export const doubled = rune(($run) => {
    $run(base);
    return base * 2;
  });
</script>

<p>Double of {base} is {$doubled}</p>

Consumers of this component can now use {$doubled} just like any other reactive value. If you need to forward a Rune from a child to a parent, simply re‑export it.

Binding to Runes

• Two‑way bindings still work with plain variables; you cannot bind directly to a Rune because it’s read‑only.
• When you need a bindable value that also participates in a Rune, keep the raw variable separate and reference it inside the Rune.

Step 6: Advanced Example – A Live Search Component

Let’s build a reusable live‑search component that debounces input, fetches results, and highlights matches. This example showcases Runes for debouncing, async fetching, and derived UI state.

<script>
  import { rune } from 'svelte/runes';
  import { writable } from 'svelte/store';

  export let endpoint = '/api/search';
  let term = '';
  const results = writable([]);

  // Debounce Rune – emits the latest term after 300 ms of inactivity
  const debouncedTerm = rune(($run) => {
    $run(term);
    const controller = new AbortController();
    const timeout = setTimeout(() => {
      controller.abort(); // cancel any pending fetch
    }, 300);
    return { value: term, abort: controller.abort, timeout };
  });

  // Fetch Rune – runs whenever debouncedTerm changes
  const fetchResults = rune(async ($run) => {
    const { value, abort, timeout } = $run(debouncedTerm);
    clearTimeout(timeout);
    try {
      const res = await fetch(`${endpoint}?q=${encodeURIComponent(value)}`, { signal: abort });
      const data = await res.json();
      results.set(data);
    } catch (e) {
      if (e.name !== 'AbortError') console.error(e);
    }
  });
</script>

<input bind:value={term} placeholder="Search…" />

<ul>
  {#each $results as item}
    <li>{item}</li>
  {/each}
</ul>

The component stays under 100 lines, yet it handles debouncing, cancellation, and UI updates without a single $: block. Notice how each Rune’s dependencies are declared explicitly, giving the compiler the full picture.

Step 7: Testing Runes

Unit testing Runes is straightforward because they are pure functions. Export the Rune factory, invoke it with a mock $run that records dependencies, and assert the returned value.

// doubleRune.js
import { rune } from 'svelte/runes';
export const double = (num) => rune(($run) => {
  $run(num);
  return num * 2;
});

// doubleRune.test.js
import { double } from './doubleRune';

test('double returns correct value', () => {
  const mockRun = jest.fn();
  const result = double(5);
  // Simulate the $run call inside the Rune
  const value = result.__run__(mockRun);
  expect(value).toBe(10);
  expect(mockRun).toHaveBeenCalledWith(5);
});

In most cases you won’t need to reach into the internal __run__ method; instead, render a tiny Svelte component that uses the Rune and assert the DOM output with @testing-library/svelte.

Performance Considerations

Because Runes expose dependencies up front, the Svelte compiler can eliminate unnecessary re‑evaluations. However, misuse can still cause performance regressions:

1. Don’t place heavy computations inside a Rune that runs on every tiny change (e.g., typing a single character).
2. Prefer memoization inside the Rune when the calculation is expensive and the inputs change infrequently.
3. Use runIf (a new helper) to conditionally execute a Rune only when a predicate holds.

Pro tip: Wrap expensive loops in runIf(() => condition, () => heavyCalc()) to keep the reactive graph shallow.

Migrating Large Codebases Incrementally

For monorepos or apps with hundreds of components, a full‑scale rewrite is risky. Adopt a hybrid approach:

• Enable the runes compiler flag globally but keep existing $: blocks functional.
• Create a lint rule (via eslint-plugin-svelte) that flags new $: usage, nudging developers toward Runes.
• Gradually refactor high‑traffic components first – they give the biggest performance win.

When a component is fully migrated, you can disable the legacy reactive mode in svelte.config.js to catch any lingering $: statements.

Real‑World Use Cases

Dashboard Widgets – A finance dashboard that shows live ticker data can use a Rune to derive moving averages from a store of raw prices. The Rune updates only when the price array changes, not on every UI interaction.

Form Validation – Instead of scattering $: blocks across each field, create a validate Rune that accepts the form state object and returns an error map. The form component then simply renders {$validate.errors}.

Animation Controllers – Runes can encapsulate spring physics, exposing a reactive position value that updates on every animation frame. Because the dependencies are known, the compiler can skip recomputation when the animation is paused.

Common Pitfalls and How to Avoid Them

Stale closures: If you capture a variable outside the Rune, it won’t be tracked. Always pass the variable into $run or include it as a function argument.

Side effects inside Runes: Runes should remain pure. Move DOM manipulation, timers, or network calls to separate async Runes or use the onMount lifecycle hook.

Over‑nesting: While nesting Runes is powerful, deep nesting can make debugging harder. Keep the hierarchy shallow and extract complex logic into helper functions.