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<script module>
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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.

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Raw state
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<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>