Skip to main content
Basic Svelte
Introduction
Reactivity
Props
Logic
Events
Bindings
Classes and styles
Actions
Transitions
Advanced Svelte
Advanced reactivity
Reusing content
Motion
Advanced bindings
Advanced transitions
Context API
Special elements
<script module>
Next steps
Basic SvelteKit
Introduction
Routing
Loading data
Headers and cookies
Shared modules
Forms
API routes
$app/state
Errors and redirects
Advanced SvelteKit
Hooks
Page options
Link options
Advanced routing
Advanced loading
Environment variables
Conclusion
Advanced Svelte Advanced reactivity Reactive classes

It’s not just variables that can be made reactive — in Svelte, we can also make properties of classes reactive.

Let’s make the width and height properties of our Box class reactive:

App
class Box {
	width = $state(0);
	height = $state(0);
	area = 0;

	// ...
}

Now, when we interact with the range inputs or click the ‘embiggen’ button, the box reacts.

We can also use $derived, so that box.area updates reactively:

App
class Box {
	width = $state(0);
	height = $state(0);
	area = $derived(this.width * this.height);

	// ...
}

In addition to $state and $derived, you can use $state.raw and $derived.by to define reactive fields.

Edit this page on GitHub

previous next
Raw state
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
<script>
	const MAX_SIZE = 200;
 
	class Box {
		width = 0;
		height = 0;
		area = 0;
 
		constructor(width, height) {
			this.width = width;
			this.height = height;
		}
 
		embiggen(amount) {
			this.width += amount;
			this.height += amount;
		}
	}
 
	const box = new Box(100, 100);
</script>
 
<label>
	<input type="range" bind:value={box.width} min={0} max={MAX_SIZE} />
	{box.width}
</label>
 
<label>
	<input type="range" bind:value={box.height} min={0} max={MAX_SIZE} />
	{box.height}
</label>
 
<button onclick={() => box.embiggen(10)}>embiggen</button>
 
<hr>
 
<div
	class="box"
	style:width="{box.width}px"
	style:height="{box.height}px"
>
	{box.area}
</div>
 
<style>
	label {
		display: flex;
		align-items: center;
	}
 
	hr {
		margin: 1em 0;
		border: none;
		border-bottom: 1px solid #888;
	}
 
	.box {
		background: radial-gradient(at 25% 25%, hsl(15 100 60), hsl(15 100 50)) ;
		border-radius: 2px;
		filter: drop-shadow(0 0 10px hsl(15 100 50 / 0.3));
		display: flex;
		align-items: center;
		justify-content: center;
		overflow: hidden;
	}
</style>