Reactive Architecture Comparison: Svelte Runes vs React Signals

If you’ve been keeping an ear to the ground in the frontend world lately, you’ve almost certainly noticed a shift. For years, we’ve been building websites a certain way, and then suddenly, everyone starts talking about "Signals," "Runes," and "Fine-Grained Reactivity." It sounds a bit like a fantasy novel, doesn't it? But really, it's just the next natural step in how we handle data in our applications.

In this guide, I want to break down the big comparison happening right now: Svelte 5 Runes vs React 19 Signals.

I have to be honest with you right out of the gate—React doesn't officially call them signals (we'll get to that nuance in a minute), but the community comparison is inevitable. Both frameworks are essentially racing toward the same goal: making your apps faster and your code a little cleaner.

Whether you're just starting out or you're trying to keep up with the chaotic churn of JavaScript, I’m going to walk you through this simply. No complex jargon without an explanation. Just you, me, and the current state of the web.

1. Introduction: The Reactivity Renaissance

Let’s start with a basic question: What actually is reactivity?

Think about an Excel spreadsheet for a second. You have a cell that calculates the sum of a column (say, Cell C1 = A1 + B1). If you change the number in A1, C1 updates automatically. You don't have to tell C1 to refresh; it just knows. That is reactivity.

In web development, we want our user interfaces (UI) to behave exactly like that spreadsheet. When a user updates their profile name, the header of the website should update instantly to match.

For a long time, frameworks like React and Svelte handled this quite differently. But recently, it feels like we’ve entered what I call the Reactivity Renaissance.

Why now? Honestly, because our apps are getting huge. We used to be okay with re-rendering entire sections of a page just to change one tiny number. It was a bit like repainting your entire living room just because you hung up a new picture frame. It works, sure, but it’s wasteful.

Both Svelte 5 and React 19 arThe frontend world is currently witnessing a massive shift in how we handle state reactivity. For years, React's virtual DOM diffing was the gold standard. But now, "Signals" are taking over, promising fine-grained reactivity without the overhead of re-rendering entire component trees. Svelte 5 introduces "Runes" to fully embrace this model, while React 19 is doubling down on its own compiler-based approach. Which one is the future? Let's dive in. For a deep dive into optimizing current React apps, see our React Performance Guide.

2. What Are Svelte 5 Runes?

Svelte 5's Runes are a direct response to the "React Hook usage rules" complexity. There are no "rules of Runes" like there are "rules of Hooks" (don't call inside loops, etc.). You just use them. React 19 simplifies things by removing useMemo and useCallback via the compiler, but the underlying mental model of "render cycles" remains. For now, mastering React Hooks is still essential for React developers.t of its work on your computer before you even deploy the website. It’s a compiler.

In previous versions (Svelte 3 and 4), reactivity was a bit magical. You just declared a variable with let, and Svelte figured out the rest. It was easy, but sometimes it felt a little too magic. As apps got complex, it became surprisingly hard to tell what was reactive and what wasn't.

Enter Svelte 5 Runes.

Think of a Rune as a special marker—like a highlighter pen—that you use to tell Svelte, "Hey, pay attention to this specific variable. If it changes, update the screen."

The Magic Symbols

Svelte 5 introduces a syntax that looks like function calls starting with a dollar sign ($). These are the Runes.

• $state: This creates a reactive value.
• $derived: This creates a value that depends on other values (like that spreadsheet sum).
• $effect: This runs code when things change (side effects).

Here is the beauty of it: It’s explicit. You don't have to guess anymore. If you see a $, you know reactivity is happening there.

<script>
  // This is a Rune!
  let count = $state(0);
  
  // This is a derived value. It updates automatically when count changes.
  let double = $derived(count * 2);

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

<button onclick={increment}>
  Clicked {count} times (Double: {double})
</button>

In the example above, Svelte knows exactly where count is used. When you click that button, Svelte surgically updates the text node in the DOM. It doesn't check the rest of the page. It just snipes that one number.

This approach is heavily inspired by the "Signals" pattern popularized by other frameworks like SolidJS. Svelte just put their own unique "compiler flavor" on it and called them Runes.

3. Explaining React 19's Reactivity Model

Now, let's talk about the giant in the room: React.

If you go looking for "React Signals" in the official documentation, you won't find it. The React team has taken a different philosophical path here. While the rest of the world seems to be rushing toward explicit Signals (like Svelte's Runes), React wants to keep things looking like standard JavaScript.

However, the problem React 19 solves is the exact same one Svelte Runes solves: preventing unnecessary re-renders.

The "Old" React Problem

Historically, when you changed state in a React component (using useState), React would re-render that component and all of its children. It’s a bit like a waterfall. If the water starts at the top, everything below gets wet.

To stop this, developers had to use tools called useMemo and useCallback. It was manual labor. You basically had to tell React, "Please don't re-render this child unless this specific data changes." It was tedious and easy to mess up. We've previously covered the basics of these hooks in our article on The Complete Guide to React Hooks, which details just how much mental overhead this used to cause.

The React 19 Solution: The Compiler

InstReact 19 doesn't use "signals" in the way SolidJS or Svelte do. instead, it relies on the React Compiler (formerly React Forget) to automatically memoize components and values. You write normal React code, and the compiler figures out the optimal dependency graph. It's "automatic reactivity" rather than "fine-grained reactivity." We cover the compiler in our Next.js 16/React 19 Review.s of your UI depend on what data. It effectively "memoizes" (caches) everything by default.

So, while Svelte gives you manual controls (Runes) to define reactivity, React says, "Don't worry about it. Write normal code, and our compiler will figure out the fine-grained updates for you."

Technically, under the hood, React isn't using "Signals" in the strict architectural sense (it still uses a Virtual DOM), but the outcome for the developer is similar: you stop worrying about performance hacks.

4. Syntax Showdown: Runes vs. Signals

Let’s get our hands dirty. How does the code actually look when you compare them side-by-side?

We are going to build the "Hello World" of reactivity: a counter that also tracks if the number is even or odd.

Svelte 5 (Runes)

In Svelte 5, you have to explicitly opt-in to reactivity using the $state rune.

<script>
  let count = $state(0);
  // $derived ensures this updates ONLY when count changes
  let isEven = $derived(count % 2 === 0);

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

<button onclick={handleClick}>
  Count is {count}
</button>
<p>The number is {isEven ? 'Even' : 'Odd'}</p>

The Vibe: It reads like modern JavaScript with a few special keywords thrown in. It feels very precise. You declare exactly what you want to happen, and it happens.

React 19 (With Compiler)

In React 19, you stick to the hooks you know, but you can delete all those manual optimizations you used to write.

import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);
  
  // In React 19, the compiler automatically memoizes this!
  // No need for useMemo(() => count % 2 === 0, [count])
  const isEven = count % 2 === 0;

  function handleClick() {
    setCount(count + 1);
  }

  return (
    <>
      <button onClick={handleClick}>
        Count is {count}
      </button>
      <p>The number is {isEven ? 'Even' : 'Odd'}</p>
    </>
  );
}

The Vibe: It looks incredibly clean. If you are coming from older React versions, it feels like a weight has been lifted off your shoulders. You just write the logic, and the framework handles the performance bits.

Key Difference

In Svelte, you are explicitly defining the dependency graph using $derived. In React, the compiler infers the dependency graph for you. Svelte is explicit; React is implicit.

5. Under the Hood: Compiler vs. Virtual DOM

This is where things get a bit technical, but stick with me—it’s actually pretty fascinating.

Svelte's Approach: "No Virtual DOM"

Svelte has famously campaigned against the Virtual DOM (VDOM). The VDOM is basically a copy of your UI kept in memory. When data changes, the framework compares the copy to the real UI (diffing) and applies changes.

Svelte 5 Runes bypass this entirely. Because you marked your variables with $state, the Svelte compiler generates code that says, "When count changes, go directly to this text node in the HTML and update it."

It’s surgical. It’s like a sniper rifle. There is no "diffing." There is only "doing."

React's Approach: "Optimized Virtual DOM"

React keeps the Virtual DOM. Many people thought React might ditch it in version 19, but they didn't.

Instead, the React Compiler makes the VDOM much faster. Before, React would waste time checking parts of the tree that hadn't changed. Now, the Compiler marks parts of the tree as "static."

Imagine you have a list of 100 items and a header that says "My List." In the old React, if you added an item, React might check the header again just to be safe. In React 19, the Compiler flags the header as static. React knows, "I never need to check that header again."

So, while React isn't strictly "fine-grained" in the same way Svelte is (it still renders components), the Compiler simulates that efficiency by skipping all the unnecessary work.

6. Performance Implications

So, who actually wins the speed race?

If we look at raw benchmarks, Svelte and its fine-grained Runes usually have a slight edge in startup time and memory usage. This is simply because Svelte doesn't have the overhead of the Virtual DOM library code to load. The browser has less JavaScript to download and execute.

However, for 99% of applications, you will not notice the difference.

React 19 has closed the gap significantly. By removing the "wasted renders" that plagued React 18, the user experience is snappy.

When does it matter?

• Low-end Mobile Devices: Svelte shines here. Less JavaScript means less battery drain and faster load times on cheap Android phones.
• Data-Heavy Dashboards: If you have a stock trading app with thousands of numbers updating per second, Svelte’s direct DOM manipulation (Signals/Runes) is theoretically more efficient than React’s VDOM diffing.

But here is the kicker: Architecture matters more than the framework. If you write bad code, both frameworks will be slow. If you write good code, both will be instant.

7. Developer Experience and Learning Curve

This is the most subjective part, but often the most important. Which one feels better to write?

The Case for Svelte 5

Svelte has always been praised for being beginner-friendly. Svelte 5 adds a little bit of complexity with the Runes syntax (you have to learn what $state and $derived do), but it really clarifies the mental model.

The "Magic" of Svelte 3/4 was confusing because you didn't always know why things updated. With Runes, the rules are clear. It feels robust. It feels like you are building engineering systems, not just hacking together a script.

The Case for React 19

React 19 is a massive win for Developer Experience (DX). The biggest complaint about React for the last five years was "Why do I have to use useMemo everywhere?"

That pain is gone.

React 19 feels like "Just JavaScript" again. You write functions, you return UI. It’s incredibly freeing. However, the ecosystem is still massive. You still need to learn about hooks, contexts, and the component lifecycle.

The "Mental Shift"

If you are coming from a traditional programming background (like Java or C#), Svelte's Runes might feel more logical. You have a variable, and you subscribe to its changes.

If you are a functional programmer who loves the idea of UI as a function of state UI = f(state), React remains the purest implementation of that idea.

8. Conclusion: The Future of UI State

We are living in a fascinating time. For years, frameworks diverged. React went one way (VDOM), Svelte and Solid went another (Signals/Compiler).

Now, it seems they are converging.

• Svelte is adopting explicit reactivity (Runes) to handle complex apps better.
• React is adopting a Compiler to automate performance, mimicking the benefits of fine-grained reactivity.

The line between "Svelte 5 Runes vs React 19 Signals" is blurry because the goal is the same: effortless performance.

So, which one should you choose?

• Choose Svelte 5 if you want a lightweight bundle, explicit control over reactivity, and a framework that feels like a modern, souped-up HTML.
• Choose React 19 if you want the massive ecosystem, the job market stability, and a "set it and forget it" performance model thanks to the new Compiler.

There is no wrong answer here. The fact that we have two incredible, high-performance options pushing each other to be better? That’s the real win for us developers.

Frequently Asked Questions

Do I need to rewrite my Svelte 4 app to use Runes in Svelte 5? No! Svelte 5 is backward compatible. You can mix and match the old syntax with the new Runes syntax while you migrate gradually. However, Runes are definitely the recommended way forward for new components.

Does React 19 actually use Signals? Not in the way Svelte or Solid does. React 19 uses a Compiler to achieve fine-grained reactivity behavior, but it doesn't expose a Signal primitive to the developer. You still use useState, but the updates are much smarter.

Which one is harder to learn for a beginner? Generally, Svelte is considered easier to pick up because it looks more like standard HTML/CSS/JS. React requires understanding the "Rules of Hooks," though React 19 makes this significantly easier by removing the need for manual memoization.

Are Svelte Runes the same as SolidJS Signals? They are conceptually very similar. Both implement fine-grained reactivity. The main difference is syntax and how they integrate with the framework's templating system. Svelte uses the $ syntax, while Solid uses getter/setter functions.

Will React ever get rid of the Virtual DOM? It's unlikely in the near future. The React team believes the Virtual DOM offers benefits for cross-platform rendering (like React Native) and that the Compiler can optimize it enough that the overhead becomes negligible.