Runes in Svelte 5 - Full Stack SvelteKit

Svelte has been turning heads in the frontend development world for a while now. Its compile-time approach to reactivity has offered a refreshing alternative to the runtime-heavy frameworks we’ve grown accustomed to.

But as with any tool, there’s always room for improvement.

Enter Svelte 5 and its headline feature: runes .

I’ve been tinkering with Svelte since its early days, and I must say, runes represent a significant shift in how we think about reactivity in Svelte applications.

Let’s dive in and explore what makes them so intriguing.

What are Runes?

Before we get our hands dirty with code, let’s establish what we mean by “runes” in the context of Svelte.

Runes are essentially function-like symbols that provide instructions to the Svelte compile r. They’re not imports you pull in from a library, but rather part of the extended JavaScript syntax that Svelte understands.

The key runes we’ll be working with are:

$state for declaring reactive state
$derived for computed values
$effect for side effects
$props for component properties

Now, you might be thinking, “Justin, this sounds an awful lot like the reactive declarations we already have in Svelte 4.”

And you’d be right to draw that parallel. But as we’ll see, runes solve some fundamental issues that have been lurking in Svelte’s reactivity model.

The Problem Runes Solve

To appreciate the elegance of runes, we first need to understand the limitations they’re addressing. In Svelte 4, reactivity was somewhat magical - let declarations at the top level of a component were automatically reactive, and the $: label turned any statement into a reactive declaration.

This approach, while intuitive for simple cases, had some drawbacks:

It only worked at the top level of .svelte files, creating an inconsistency between component files and regular JavaScript.
It made it difficult to create reusable reactive logic outside of components.
The reactivity was coarse-grained, often causing unnecessary updates.

Let me illustrate this with a simple example:

<script>
  let count = 0;
  $: doubled = count * 2;

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

<button on:click={increment}>
  Clicks: {count} (Doubled: {doubled})
</button>

This works fine in a .svelte file, but try to move that logic into a separate .js file, and suddenly, the reactivity vanishes. It’s this inconsistency that runes aim to address.

How Runes Work

Now, let’s rewrite our counter example using runes:

<script>
  let count = $state(0);
  let doubled = $derived(count * 2);

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

<button onclick={increment}>
  Clicks: {count} (Doubled: {doubled})
</button>

At first glance, this might not look like a significant change. But the implications are profound.

By using $state and $derived, we’ve made our reactive intentions explicit. This code could now live in a .js file, and it would work the same way.

Let’s break down each rune:

$state

$state declares a piece of reactive state. Unlike a regular let declaration, it’s reactive wherever it appears, not just at the top level of a component.

$derived

$derived creates a value that depends on other reactive values. It’s similar to a $: declaration in Svelte 4, but again, it works anywhere in your code.

$effect

$effect is used for side effects. It’s similar to $: blocks in Svelte 4, but with some key differences:

$effect(() => {
  console.log(`The count is now ${count}`);
});

This will run initially and then every time count changes. Unlike $: blocks, $effect clearly separates side effects from derived values.

$props

$props is used to declare component properties:

let { initialCount = 0 } = $props();
let count = $state(initialCount);

This replaces the export let syntax in Svelte 4.

Runes in Action

Now that we’ve covered the basics, let’s look at a more complex example to see how runes can simplify our code:

// counter.svelte.js
export function createCounter(initialCount = 0) {
  let count = $state(initialCount);
  let doubled = $derived(count * 2);
  let isEven = $derived(count % 2 === 0);

  function increment() {
    count += 1;
  }

  $effect(() => {
    console.log(`Count changed to ${count}`);
  });

  return {
    get count() { return count; },
    get doubled() { return doubled; },
    get isEven() { return isEven; },
    increment
  };
}

// App.svelte
<script>
  import { createCounter } from './counter.svelte.js';

  const counter = createCounter(5);
</script>

<button onclick={counter.increment}>
  Count: {counter.count}
  (Doubled: {counter.doubled}, {counter.isEven ? 'Even' : 'Odd'})
</button>

This example demonstrates how runes enable us to create reusable reactive logic outside of components. The createCounter function encapsulates all the reactive behavior, which can now be easily shared across multiple components or even applications.

The Benefits of Runes

As we’ve seen, runes bring several significant benefits:

Universal reactivity : The same reactive logic works everywhere, not just in .svelte files.
Improved code organization : Reactive logic can be easily extracted and reused.
Fine-grained reactivity : Updates are more targeted, potentially improving performance.
Explicit intentions : The code clearly shows what’s meant to be reactive.

Migrating to Runes

Now, you might be wondering, “Do I need to rewrite all my Svelte 4 code to use runes?”

The short answer is no. Svelte 5 is designed to be backwards compatible, so your existing Svelte 4 code should continue to work.

That said, I’d recommend gradually adopting runes in new code and when refactoring existing code.

Start with the most complex components or those with performance issues - these are likely to benefit most from the fine-grained reactivity that runes enable.

Potential Drawbacks

Of course, no feature comes without trade-offs.

Runes introduce a new concept to learn, which might steepen the learning curve for Svelte beginners. They also change some established Svelte patterns, which could be disorienting for experienced Svelte developers.

Moreover, the success of runes will depend on community adoption. If the community embraces runes, we’ll likely see new patterns and best practices emerge. If not, we might end up with a fractured ecosystem.

Conclusion

Runes in Svelte 5 represent a significant evolution in how we handle reactivity in Svelte applications. They solve real problems that have been lurking in Svelte’s reactivity model, and they do so in a way that feels natural and powerful.

As with any new feature, it will take time for the community to fully explore and understand the implications of runes. But from what I’ve seen so far, I’m excited about the possibilities they open up.

If you’re working with Svelte, I’d strongly encourage you to give runes a try in your next project. They might just change the way you think about reactivity in frontend development.

Remember, the best way to understand a new feature is to use it. So fire up your editor, create a new Svelte 5 project, and start experimenting with runes. Happy coding!