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SvelteRunes

$state

See also

The $state rune allows you to create reactive state, which means that your UI reacts when it changes.

<script>
	let count = $state(0);
</script>

<button onclick={() => count++}>
	clicks: {count}
</button>

Unlike other frameworks you may have encountered, there is no API for interacting with state — count is just a number, rather than an object or a function, and you can update it like you would update any other variable.

Deep state

If $state is used with an array or a simple object, the result is a deeply reactive state proxy. Proxies allow Svelte to run code when you read or write properties, including via methods like array.push(...), triggering granular updates.

State is proxified recursively until Svelte finds something other than an array or simple object (like a class or an object created with Object.create). In a case like this...

let todos = $state([
	{
		done: false,
		text: 'add more todos'
	}
]);

...modifying an individual todo’s property will trigger updates to anything in your UI that depends on that specific property:

todos[0].done = !todos[0].done;

If you push a new object to the array, it will also be proxified:

todos.push({
	done: false,
	text: 'eat lunch'
});

When you update properties of proxies, the original object is not mutated. If you need to use your own proxy handlers in a state proxy, you should wrap the object after wrapping it in $state.

Note that if you destructure a reactive value, the references are not reactive — as in normal JavaScript, they are evaluated at the point of destructuring:

let { done, text } = todos[0];
// this will not affect the value of `done`todos[0].done = !todos[0].done;

Classes

Class instances are not proxied. Instead, you can use $state in class fields (whether public or private), or as the first assignment to a property immediately inside the constructor:

class Todo {
	done = $state(false);

	constructor(text) {
		this.text = $state(text);
	}

	reset() {
		this.text = '';
		this.done = false;
	}
}

The compiler transforms done and text into get / set methods on the class prototype referencing private fields. This means the properties are not enumerable.

When calling methods in JavaScript, the value of this matters. This won’t work, because this inside the reset method will be the <button> rather than the Todo:

<button onclick={todo.reset}>
	reset
</button>

You can either use an inline function...

<button onclick={() => todo.reset()}>
	reset
</button>

...or use an arrow function in the class definition:

class Todo {
	done = $state(false);

	constructor(text) {
		this.text = $state(text);
	}

	reset = () => {
		this.text = '';
		this.done = false;
	}
}

Built-in classes

Svelte provides reactive implementations of built-in classes like Set, Map, Date and URL that can be imported from svelte/reactivity.

$state.raw

In cases where you don’t want objects and arrays to be deeply reactive you can use $state.raw.

State declared with $state.raw cannot be mutated; it can only be reassigned. In other words, rather than assigning to a property of an object, or using an array method like push, replace the object or array altogether if you’d like to update it:

let person = $state.raw({
	name: 'Heraclitus',
	age: 49
});
// this will have no effectperson.age += 1;
// this will work, because we're creating a new personperson = {
	name: 'Heraclitus',
	age: 50
};

This can improve performance with large arrays and objects that you weren’t planning to mutate anyway, since it avoids the cost of making them reactive. Note that raw state can contain reactive state (for example, a raw array of reactive objects).

As with $state, you can declare class fields using $state.raw.

$state.snapshot

To take a static snapshot of a deeply reactive $state proxy, use $state.snapshot:

<script>
	let counter = $state({ count: 0 });

	function onclick() {
		// Will log `{ count: ... }` rather than `Proxy { ... }`
		console.log($state.snapshot(counter));
	}
</script>

This is handy when you want to pass some state to an external library or API that doesn’t expect a proxy, such as structuredClone.

If a value has a toJSON method, the snapshot will clone the value returned from toJSON instead of the original object.

$state.eager

When state changes, it may not be reflected in the UI immediately if it is used by an await expression, because updates are synchronized.

In some cases, you may want to update the UI as soon as the state changes. For example, you might want to update a navigation bar when the user clicks on a link, so that they get visual feedback while waiting for the new page to load. To do this, use $state.eager(value):

<nav>
	<a href="/" aria-current={$state.eager(pathname) === '/' ? 'page' : null}>home</a>
	<a href="/about" aria-current={$state.eager(pathname) === '/about' ? 'page' : null}>about</a>
</nav>

Use this feature sparingly, and only to provide feedback in response to user action — in general, allowing Svelte to coordinate updates will provide a better user experience.

Passing state into functions

JavaScript is a pass-by-value language — when you call a function, the arguments are the values rather than the variables. In other words:

index
/**
 * @param {number} a
 * @param {number} b */function add(a, b) {
	return a + b;
}

let a = 1;
let b = 2;
let total = add(a, b);
console.log(total); // 3

a = 3;
b = 4;
console.log(total); // still 3!
function add(a: number, b: number) {
	return a + b;
}

let a = 1;
let b = 2;
let total = add(a, b);
console.log(total); // 3

a = 3;
b = 4;
console.log(total); // still 3!

If add wanted to have access to the current values of a and b, and to return the current total value, you would need to use functions instead:

index
/**
 * @param {() => number} getA
 * @param {() => number} getB */function add(getA, getB) {
	return () => getA() + getB();
}

let a = 1;
let b = 2;
let total = add(() => a, () => b);
console.log(total()); // 3

a = 3;
b = 4;
console.log(total()); // 7
function add(getA: () => number, getB: () => number) {
	return () => getA() + getB();
}

let a = 1;
let b = 2;
let total = add(() => a, () => b);
console.log(total()); // 3

a = 3;
b = 4;
console.log(total()); // 7

State in Svelte is no different — when you reference something declared with the $state rune...

let a = $state(1);
let b = $state(2);

...you’re accessing its current value.

Note that ‘functions’ is broad — it encompasses properties of proxies and get/set properties...

index
/**
 * @param {{ a: number, b: number }} input
 */function add(input) {
	return {
		get value() {
			return input.a + input.b;
		}
	};
}

let input = $state({ a: 1, b: 2 });
let total = add(input);
console.log(total.value); // 3

input.a = 3;
input.b = 4;
console.log(total.value); // 7
function add(input: { a: number, b: number }) {
	return {
		get value() {
			return input.a + input.b;
		}
	};
}

let input = $state({ a: 1, b: 2 });
let total = add(input);
console.log(total.value); // 3

input.a = 3;
input.b = 4;
console.log(total.value); // 7

...though if you find yourself writing code like that, consider using classes instead.

Passing state across modules

You can declare state in .svelte.js and .svelte.ts files, but you can only export that state if it’s not directly reassigned. In other words you can’t do this:

state.svelte
export let count = $state(0);

export function increment() {
	count += 1;
}

That’s because every reference to count is transformed by the Svelte compiler — the code above is roughly equivalent to this:

state.svelte
export let count = $.state(0);

export function increment() {
	$.set(count, $.get(count) + 1);
}

You can see the code Svelte generates by clicking the ‘JS Output’ tab in the playground.

Since the compiler only operates on one file at a time, if another file imports count Svelte doesn’t know that it needs to wrap each reference in $.get and $.set:

import { count } from './state.svelte.js';

console.log(typeof count); // 'object', not 'number'

This leaves you with two options for sharing state between modules — either don’t reassign it...

// This is allowed — since we're updating
// `counter.count` rather than `counter`,
// Svelte doesn't wrap it in `$.state`
export const counter = $state({
	count: 0
});

export function increment() {
	counter.count += 1;
}

...or don’t directly export it:

let count = $state(0);

export function getCount() {
	return count;
}

export function increment() {
	count += 1;
}

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