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<SYSTEM>This is the developer documentation for Svelte.</SYSTEM>
# Overview
Svelte is a framework for building user interfaces on the web. It uses a compiler to turn declarative components written in HTML, CSS and JavaScript...
```svelte
<!--- file: App.svelte --->
<script>
function greet() {
alert('Welcome to Svelte!');
}
</script>
<button onclick={greet}>click me</button>
<style>
button {
font-size: 2em;
}
</style>
```
...into lean, tightly optimized JavaScript.
You can use it to build anything on the web, from standalone components to ambitious full stack apps (using Svelte's companion application framework, [SvelteKit](../kit)) and everything in between.
These pages serve as reference documentation. If you're new to Svelte, we recommend starting with the [interactive tutorial](/tutorial) and coming back here when you have questions.
You can also try Svelte online in the [playground](/playground) or, if you need a more fully-featured environment, on [StackBlitz](https://sveltekit.new).
# Getting started
We recommend using [SvelteKit](../kit), which lets you [build almost anything](../kit/project-types). It's the official application framework from the Svelte team and powered by [Vite](https://vite.dev/). Create a new project with:
```bash
npx sv create myapp
cd myapp
npm install
npm run dev
```
Don't worry if you don't know Svelte yet! You can ignore all the nice features SvelteKit brings on top for now and dive into it later.
## Alternatives to SvelteKit
You can also use Svelte directly with Vite by running `npm create vite@latest` and selecting the `svelte` option. With this, `npm run build` will generate HTML, JS, and CSS files inside the `dist` directory using [vite-plugin-svelte](https://github.com/sveltejs/vite-plugin-svelte). In most cases, you will probably need to [choose a routing library](faq#Is-there-a-router) as well.
>[!NOTE] Vite is often used in standalone mode to build [single page apps (SPAs)](../kit/glossary#SPA), which you can also [build with SvelteKit](../kit/single-page-apps).
There are also plugins for [Rollup](https://github.com/sveltejs/rollup-plugin-svelte), [Webpack](https://github.com/sveltejs/svelte-loader) [and a few others](https://sveltesociety.dev/packages?category=build-plugins), but we recommend Vite.
## Editor tooling
The Svelte team maintains a [VS Code extension](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode), and there are integrations with various other [editors](https://sveltesociety.dev/resources#editor-support) and tools as well.
You can also check your code from the command line using [sv check](https://github.com/sveltejs/cli).
## Getting help
Don't be shy about asking for help in the [Discord chatroom](/chat)! You can also find answers on [Stack Overflow](https://stackoverflow.com/questions/tagged/svelte).
# .svelte files
Components are the building blocks of Svelte applications. They are written into `.svelte` files, using a superset of HTML.
All three sections — script, styles and markup — are optional.
<!-- prettier-ignore -->
```svelte
/// file: MyComponent.svelte
<script module>
// module-level logic goes here
// (you will rarely use this)
</script>
<script>
// instance-level logic goes here
</script>
<!-- markup (zero or more items) goes here -->
<style>
/* styles go here */
</style>
```
## `<script>`
A `<script>` block contains JavaScript (or TypeScript, when adding the `lang="ts"` attribute) that runs when a component instance is created. Variables declared (or imported) at the top level can be referenced in the component's markup.
In addition to normal JavaScript, you can use _runes_ to declare [component props]($props) and add reactivity to your component. Runes are covered in the next section.
<!-- TODO describe behaviour of `export` -->
## `<script module>`
A `<script>` tag with a `module` attribute runs once when the module first evaluates, rather than for each component instance. Variables declared in this block can be referenced elsewhere in the component, but not vice versa.
```svelte
<script module>
let total = 0;
</script>
<script>
total += 1;
console.log(`instantiated ${total} times`);
</script>
```
You can `export` bindings from this block, and they will become exports of the compiled module. You cannot `export default`, since the default export is the component itself.
> [!NOTE] If you are using TypeScript and import such exports from a `module` block into a `.ts` file, make sure to have your editor setup so that TypeScript knows about them. This is the case for our VS Code extension and the IntelliJ plugin, but in other cases you might need to setup our [TypeScript editor plugin](https://www.npmjs.com/package/typescript-svelte-plugin).
> [!LEGACY]
> In Svelte 4, this script tag was created using `<script context="module">`
## `<style>`
CSS inside a `<style>` block will be scoped to that component.
```svelte
<style>
p {
/* this will only affect <p> elements in this component */
color: burlywood;
}
</style>
```
For more information, head to the section on [styling](scoped-styles).
# .svelte.js and .svelte.ts files
Besides `.svelte` files, Svelte also operates on `.svelte.js` and `.svelte.ts` files.
These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app (though note that you [cannot export reassigned state]($state#Passing-state-across-modules)).
> [!LEGACY]
> This is a concept that didn't exist prior to Svelte 5
# What are runes?
> [!NOTE] **rune** /ruːn/ _noun_
>
> A letter or mark used as a mystical or magic symbol.
Runes are symbols that you use in `.svelte` and `.svelte.js`/`.svelte.ts` files to control the Svelte compiler. If you think of Svelte as a language, runes are part of the syntax — they are _keywords_.
Runes have a `$` prefix and look like functions:
```js
let message = $state('hello');
```
They differ from normal JavaScript functions in important ways, however:
- You don't need to import them — they are part of the language
- They're not values — you can't assign them to a variable or pass them as arguments to a function
- Just like JavaScript keywords, they are only valid in certain positions (the compiler will help you if you put them in the wrong place)
> [!LEGACY]
> Runes didn't exist prior to Svelte 5.
# $state
The `$state` rune allows you to create _reactive state_, which means that your UI _reacts_ when it changes.
```svelte
<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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) allow Svelte to run code when you read or write properties, including via methods like `array.push(...)`, triggering granular updates.
> [!NOTE] Classes like `Set` and `Map` will not be proxied, but Svelte provides reactive implementations for various built-ins like these that can be imported from [`svelte/reactivity`](./svelte-reactivity).
State is proxified recursively until Svelte finds something other than an array or simple object. In a case like this...
```js
let todos = $state([
{
done: false,
text: 'add more todos'
}
]);
```
...modifying an individual todo's property will trigger updates to anything in your UI that depends on that specific property:
```js
let todos = [{ done: false, text: 'add more todos' }];
// ---cut---
todos[0].done = !todos[0].done;
```
If you push a new object to the array, it will also be proxified:
```js
let todos = [{ done: false, text: 'add more todos' }];
// ---cut---
todos.push({
done: false,
text: 'eat lunch'
});
```
> [!NOTE] When you update properties of proxies, the original object is _not_ mutated.
Note that if you destructure a reactive value, the references are not reactive — as in normal JavaScript, they are evaluated at the point of destructuring:
```js
let todos = [{ done: false, text: 'add more todos' }];
// ---cut---
let { done, text } = todos[0];
// this will not affect the value of `done`
todos[0].done = !todos[0].done;
```
### Classes
You can also use `$state` in class fields (whether public or private):
```js
// @errors: 7006 2554
class Todo {
done = $state(false);
text = $state();
constructor(text) {
this.text = text;
}
reset() {
this.text = '';
this.done = false;
}
}
```
> [!NOTE] 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`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) matters. This won't work, because `this` inside the `reset` method will be the `<button>` rather than the `Todo`:
```svelte
<button onclick={todo.reset}>
reset
</button>
```
You can either use an inline function...
```svelte
<button onclick=+++{() => todo.reset()}>+++
reset
</button>
```
...or use an arrow function in the class definition:
```js
// @errors: 7006 2554
class Todo {
done = $state(false);
text = $state();
constructor(text) {
this.text = text;
}
+++reset = () => {+++
this.text = '';
this.done = false;
}
}
```
## `$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:
```js
let person = $state.raw({
name: 'Heraclitus',
age: 49
});
// this will have no effect
person.age += 1;
// this will work, because we're creating a new person
person = {
name: 'Heraclitus',
age: 50
};
```
This can improve performance with large arrays and objects that you weren't planning to mutate anyway, since it avoids the cost of making them reactive. Note that raw state can _contain_ reactive state (for example, a raw array of reactive objects).
## `$state.snapshot`
To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snapshot`:
```svelte
<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`.
## 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:
```js
/// file: index.js
// @filename: index.js
// ---cut---
/**
* @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!
```
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:
```js
/// file: index.js
// @filename: index.js
// ---cut---
/**
* @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
```
State in Svelte is no different — when you reference something declared with the `$state` rune...
```js
let a = +++$state(1)+++;
let b = +++$state(2)+++;
```
...you're accessing its _current value_.
Note that 'functions' is broad — it encompasses properties of proxies and [`get`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get)/[`set`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set) properties...
```js
/// file: index.js
// @filename: index.js
// ---cut---
/**
* @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
```
...though if you find yourself writing code like that, consider using [classes](#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:
```js
/// file: state.svelte.js
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:
```js
/// file: state.svelte.js (compiler output)
// @filename: index.ts
interface Signal<T> {
value: T;
}
interface Svelte {
state<T>(value?: T): Signal<T>;
get<T>(source: Signal<T>): T;
set<T>(source: Signal<T>, value: T): void;
}
declare const $: Svelte;
// ---cut---
export let count = $.state(0);
export function increment() {
$.set(count, $.get(count) + 1);
}
```
> [!NOTE] You can see the code Svelte generates by clicking the 'JS Output' tab in the [playground](/playground).
Since the compiler only operates on one file at a time, if another file imports `count` Svelte doesn't know that it needs to wrap each reference in `$.get` and `$.set`:
```js
// @filename: state.svelte.js
export let count = 0;
// @filename: index.js
// ---cut---
import { count } from './state.svelte.js';
console.log(typeof count); // 'object', not 'number'
```
This leaves you with two options for sharing state between modules — either don't reassign it...
```js
// This is allowed — since we're updating
// `counter.count` rather than `counter`,
// Svelte doesn't wrap it in `$.state`
export const counter = $state({
count: 0
});
export function increment() {
counter.count += 1;
}
```
...or don't directly export it:
```js
let count = $state(0);
export function getCount() {
return count;
}
export function increment() {
count += 1;
}
```
# $derived
Derived state is declared with the `$derived` rune:
```svelte
<script>
let count = $state(0);
let doubled = $derived(count * 2);
</script>
<button onclick={() => count++}>
{doubled}
</button>
<p>{count} doubled is {doubled}</p>
```
The expression inside `$derived(...)` should be free of side-effects. Svelte will disallow state changes (e.g. `count++`) inside derived expressions.
As with `$state`, you can mark class fields as `$derived`.
> [!NOTE] Code in Svelte components is only executed once at creation. Without the `$derived` rune, `doubled` would maintain its original value even when `count` changes.
## `$derived.by`
Sometimes you need to create complex derivations that don't fit inside a short expression. In these cases, you can use `$derived.by` which accepts a function as its argument.
```svelte
<script>
let numbers = $state([1, 2, 3]);
let total = $derived.by(() => {
let total = 0;
for (const n of numbers) {
total += n;
}
return total;
});
</script>
<button onclick={() => numbers.push(numbers.length + 1)}>
{numbers.join(' + ')} = {total}
</button>
```
In essence, `$derived(expression)` is equivalent to `$derived.by(() => expression)`.
## Understanding dependencies
Anything read synchronously inside the `$derived` expression (or `$derived.by` function body) is considered a _dependency_ of the derived state. When the state changes, the derived will be marked as _dirty_ and recalculated when it is next read.
To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).
## Overriding derived values
Derived expressions are recalculated when their dependencies change, but you can temporarily override their values by reassigning them (unless they are declared with `const`). This can be useful for things like _optimistic UI_, where a value is derived from the 'source of truth' (such as data from your server) but you'd like to show immediate feedback to the user:
```svelte
<script>
let { post, like } = $props();
let likes = $derived(post.likes);
async function onclick() {
// increment the `likes` count immediately...
likes += 1;
// and tell the server, which will eventually update `post`
try {
await like();
} catch {
// failed! roll back the change
likes -= 1;
}
}
</script>
<button {onclick}>🧡 {likes}</button>
```
> [!NOTE] Prior to Svelte 5.25, deriveds were read-only.
## Deriveds and reactivity
Unlike `$state`, which converts objects and arrays to [deeply reactive proxies]($state#Deep-state), `$derived` values are left as-is. For example, [in a case like this](/playground/untitled#H4sIAAAAAAAAE4VU22rjMBD9lUHd3aaQi9PdstS1A3t5XvpQ2Ic4D7I1iUUV2UjjNMX431eS7TRdSosxgjMzZ45mjt0yzffIYibvy0ojFJWqDKCQVBk2ZVup0LJ43TJ6rn2aBxw-FP2o67k9oCKP5dziW3hRaUJNjoYltjCyplWmM1JIIAn3FlL4ZIkTTtYez6jtj4w8WwyXv9GiIXiQxLVs9pfTMR7EuoSLIuLFbX7Z4930bZo_nBrD1bs834tlfvsBz9_SyX6PZXu9XaL4gOWn4sXjeyzftv4ZWfyxubpzxzg6LfD4MrooxELEosKCUPigQCMPKCZh0OtQE1iSxcsmdHuBvCiHZXALLXiN08EL3RRkaJ_kDVGle0HcSD5TPEeVtj67O4Nrg9aiSNtBY5oODJkrL5QsHtN2cgXp6nSJMWzpWWGasdlsGEMbzi5jPr5KFr0Ep7pdeM2-TCelCddIhDxAobi1jqF3cMaC1RKp64bAW9iFAmXGIHfd4wNXDabtOLN53w8W53VvJoZLh7xk4Rr3CoL-UNoLhWHrT1JQGcM17u96oES5K-kc2XOzkzqGCKL5De79OUTyyrg1zgwXsrEx3ESfx4Bz0M5UjVMHB24mw9SuXtXFoN13fYKOM1tyUT3FbvbWmSWCZX2Er-41u5xPoml45svRahl9Wb9aasbINJixDZwcPTbyTLZSUsAvrg_cPuCR7s782_WU8343Y72Qtlb8OYatwuOQvuN13M_hJKNfxann1v1U_B1KZ_D_mzhzhz24fw85CSz2irtN9w9HshBK7AQAAA==)...
```svelte
let items = $state([...]);
let index = $state(0);
let selected = $derived(items[index]);
```
...you can change (or `bind:` to) properties of `selected` and it will affect the underlying `items` array. If `items` was _not_ deeply reactive, mutating `selected` would have no effect.
## Update propagation
Svelte uses something called _push-pull reactivity_ — when state is updated, everything that depends on the state (whether directly or indirectly) is immediately notified of the change (the 'push'), but derived values are not re-evaluated until they are actually read (the 'pull').
If the new value of a derived is referentially identical to its previous value, downstream updates will be skipped. In other words, Svelte will only update the text inside the button when `large` changes, not when `count` changes, even though `large` depends on `count`:
```svelte
<script>
let count = $state(0);
let large = $derived(count > 10);
</script>
<button onclick={() => count++}>
{large}
</button>
```
# $effect
Effects are functions that run when state updates, and can be used for things like calling third-party libraries, drawing on `<canvas>` elements, or making network requests. They only run in the browser, not during server-side rendering.
Generally speaking, you should _not_ update state inside effects, as it will make code more convoluted and will often lead to never-ending update cycles. If you find yourself doing so, see [when not to use `$effect`](#When-not-to-use-$effect) to learn about alternative approaches.
You can create an effect with the `$effect` rune ([demo](/playground/untitled#H4sIAAAAAAAAE31S246bMBD9lZF3pSRSAqTVvrCAVPUP2sdSKY4ZwJJjkD0hSVH-vbINuWxXfQH5zMyZc2ZmZLVUaFn6a2R06ZGlHmBrpvnBvb71fWQHVOSwPbf4GS46TajJspRlVhjZU1HqkhQSWPkHIYdXS5xw-Zas3ueI6FRn7qHFS11_xSRZhIxbFtcDtw7SJb1iXaOg5XIFeQGjzyPRaevYNOGZIJ8qogbpe8CWiy_VzEpTXiQUcvPDkSVrSNZz1UlW1N5eLcqmpdXUvaQ4BmqlhZNUCgxuzFHDqUWNAxrYeUM76AzsnOsdiJbrBp_71lKpn3RRbii-4P3f-IMsRxS-wcDV_bL4PmSdBa2wl7pKnbp8DMgVvJm8ZNskKRkEM_OzyOKQFkgqOYBQ3Nq89Ns0nbIl81vMFN-jKoLMTOr-SOBOJS-Z8f5Y6D1wdcR8dFqvEBdetK-PHwj-z-cH8oHPY54wRJ8Ys7iSQ3Bg3VA9azQbmC9k35kKzYa6PoVtfwbbKVnBixBiGn7Pq0rqJoUtHiCZwAM3jdTPWCVtr_glhVrhecIa3vuksJ_b7TqFs4DPyriSjd5IwoNNQaAmNI-ESfR2p8zimzvN1swdCkvJHPH6-_oX8o1SgcIDAAA=)):
```svelte
<script>
let size = $state(50);
let color = $state('#ff3e00');
let canvas;
$effect(() => {
const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
// this will re-run whenever `color` or `size` change
context.fillStyle = color;
context.fillRect(0, 0, size, size);
});
</script>
<canvas bind:this={canvas} width="100" height="100"></canvas>
```
When Svelte runs an effect function, it tracks which pieces of state (and derived state) are accessed (unless accessed inside [`untrack`](svelte#untrack)), and re-runs the function when that state later changes.
> [!NOTE] If you're having difficulty understanding why your `$effect` is rerunning or is not running see [understanding dependencies](#Understanding-dependencies). Effects are triggered differently than the `$:` blocks you may be used to if coming from Svelte 4.
### Understanding lifecycle
Your effects run after the component has been mounted to the DOM, and in a [microtask](https://developer.mozilla.org/en-US/docs/Web/API/HTML_DOM_API/Microtask_guide) after state changes. Re-runs are batched (i.e. changing `color` and `size` in the same moment won't cause two separate runs), and happen after any DOM updates have been applied.
You can use `$effect` anywhere, not just at the top level of a component, as long as it is called while a parent effect is running.
> [!NOTE] Svelte uses effects internally to represent logic and expressions in your template — this is how `<h1>hello {name}!</h1>` updates when `name` changes.
An effect can return a _teardown function_ which will run immediately before the effect re-runs ([demo](/playground/untitled#H4sIAAAAAAAAE42SQVODMBCF_8pOxkPRKq3HCsx49K4n64xpskjGkDDJ0tph-O8uINo6HjxB3u7HvrehE07WKDbiyZEhi1osRWksRrF57gQdm6E2CKx_dd43zU3co6VB28mIf-nKO0JH_BmRRRVMQ8XWbXkAgfKtI8jhIpIkXKySu7lSG2tNRGZ1_GlYr1ZTD3ddYFmiosUigbyAbpC2lKbwWJkIB8ZhhxBQBWRSw6FCh3sM8GrYTthL-wqqku4N44TyqEgwF3lmRHr4Op0PGXoH31c5rO8mqV-eOZ49bikgtcHBL55tmhIkEMqg_cFB2TpFxjtg703we6NRL8HQFCS07oSUCZi6Rm04lz1yytIHBKoQpo1w6Gsm4gmyS8b8Y5PydeMdX8gwS2Ok4I-ov5NZtvQde95GMsccn_1wzNKfu3RZtS66cSl9lvL7qO1aIk7knbJGvefdtIOzi73M4bYvovUHDFk6AcX_0HRESxnpBOW_jfCDxIZCi_1L_wm4xGQ60wIAAA==)).
```svelte
<script>
let count = $state(0);
let milliseconds = $state(1000);
$effect(() => {
// This will be recreated whenever `milliseconds` changes
const interval = setInterval(() => {
count += 1;
}, milliseconds);
return () => {
// if a teardown function is provided, it will run
// a) immediately before the effect re-runs
// b) when the component is destroyed
clearInterval(interval);
};
});
</script>
<h1>{count}</h1>
<button onclick={() => (milliseconds *= 2)}>slower</button>
<button onclick={() => (milliseconds /= 2)}>faster</button>
```
Teardown functions also run when the effect is destroyed, which happens when its parent is destroyed (for example, a component is unmounted) or the parent effect re-runs.
### Understanding dependencies
`$effect` automatically picks up any reactive values (`$state`, `$derived`, `$props`) that are _synchronously_ read inside its function body (including indirectly, via function calls) and registers them as dependencies. When those dependencies change, the `$effect` schedules a re-run.
If `$state` and `$derived` are used directly inside the `$effect` (for example, during creation of a [reactive class](https://svelte.dev/docs/svelte/$state#Classes)), those values will _not_ be treated as dependencies.
Values that are read _asynchronously_ — after an `await` or inside a `setTimeout`, for example — will not be tracked. Here, the canvas will be repainted when `color` changes, but not when `size` changes ([demo](/playground/untitled#H4sIAAAAAAAAE31T246bMBD9lZF3pWSlBEirfaEQqdo_2PatVIpjBrDkGGQPJGnEv1e2IZfVal-wfHzmzJyZ4cIqqdCy9M-F0blDlnqArZjmB3f72XWRHVCRw_bc4me4aDWhJstSlllhZEfbQhekkMDKfwg5PFvihMvX5OXH_CJa1Zrb0-Kpqr5jkiwC48rieuDWQbqgZ6wqFLRcvkC-hYvnkWi1dWqa8ESQTxFRjfQWsOXiWzmr0sSLhEJu3p1YsoJkNUcdZUnN9dagrBu6FVRQHAM10sJRKgUG16bXcGxQ44AGdt7SDkTDdY02iqLHnJVU6hedlWuIp94JW6Tf8oBt_8GdTxlF0b4n0C35ZLBzXb3mmYn3ae6cOW74zj0YVzDNYXRHFt9mprNgHfZSl6mzml8CMoLvTV6wTZIUDEJv5us2iwMtiJRyAKG4tXnhl8O0yhbML0Wm-B7VNlSSSd31BG7z8oIZZ6dgIffAVY_5xdU9Qrz1Bnx8fCfwtZ7v8Qc9j3nB8PqgmMWlHIID6-bkVaPZwDySfWtKNGtquxQ23Qlsq2QJT0KIqb8dL0up6xQ2eIBkAg_c1FI_YqW0neLnFCqFpwmreedJYT7XX8FVOBfwWRhXstZrSXiwKQjUhOZeMIleb5JZfHWn2Yq5pWEpmR7Hv-N_wEqT8hEEAAA=)):
```ts
// @filename: index.ts
declare let canvas: {
width: number;
height: number;
getContext(type: '2d', options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2D;
};
declare let color: string;
declare let size: number;
// ---cut---
$effect(() => {
const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
// this will re-run whenever `color` changes...
context.fillStyle = color;
setTimeout(() => {
// ...but not when `size` changes
context.fillRect(0, 0, size, size);
}, 0);
});
```
An effect only reruns when the object it reads changes, not when a property inside it changes. (If you want to observe changes _inside_ an object at dev time, you can use [`$inspect`]($inspect).)
```svelte
<script>
let state = $state({ value: 0 });
let derived = $derived({ value: state.value * 2 });
// this will run once, because `state` is never reassigned (only mutated)
$effect(() => {
state;
});
// this will run whenever `state.value` changes...
$effect(() => {
state.value;
});
// ...and so will this, because `derived` is a new object each time
$effect(() => {
derived;
});
</script>
<button onclick={() => (state.value += 1)}>
{state.value}
</button>
<p>{state.value} doubled is {derived.value}</p>
```
An effect only depends on the values that it read the last time it ran. This has interesting implications for effects that have conditional code.
For instance, if `condition` is `true` in the code snippet below, the code inside the `if` block will run and `color` will be evaluated. As such, changes to either `condition` or `color` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE21RQW6DMBD8ytaNBJHaJFLViwNIVZ8RcnBgXVk1xsILTYT4e20TQg89IOPZ2fHM7siMaJBx9tmaWpFqjQNlAKXEihx7YVJpdIyfRkY3G4gB8Pi97cPanRtQU8AuwuF_eNUaQuPlOMtc1SlLRWlKUo1tOwJflUikQHZtA0klzCDc64Imx0ANn8bInV1CDhtHgjClrsftcSXotluLybOUb3g4JJHhOZs5WZpuIS9gjNqkJKQP5e2ClrR4SMdZ13E4xZ8zTPOTJU2A2uE_PQ9COCI926_hTVarIU4hu_REPlBrKq2q73ycrf1N-vS4TMUsulaVg3EtR8H9rFgsg8uUsT1B2F9eshigZHBRpuaD0D3mY8Qm2BfB5N2YyRzdNEYVDy0Ja-WsFjcOUuP1HvFLWA6H3XuHTUSmmDV2--0TXonxsKbp7G9C6R__NONS-MFNvxj_d6mBAgAA).
Conversely, if `condition` is `false`, `color` will not be evaluated, and the effect will _only_ re-run again when `condition` changes.
```ts
// @filename: ambient.d.ts
declare module 'canvas-confetti' {
interface ConfettiOptions {
colors: string[];
}
function confetti(opts?: ConfettiOptions): void;
export default confetti;
}
// @filename: index.js
// ---cut---
import confetti from 'canvas-confetti';
let condition = $state(true);
let color = $state('#ff3e00');
$effect(() => {
if (condition) {
confetti({ colors: [color] });
} else {
confetti();
}
});
```
## `$effect.pre`
In rare cases, you may need to run code _before_ the DOM updates. For this we can use the `$effect.pre` rune:
```svelte
<script>
import { tick } from 'svelte';
let div = $state();
let messages = $state([]);
// ...
$effect.pre(() => {
if (!div) return; // not yet mounted
// reference `messages` array length so that this code re-runs whenever it changes
messages.length;
// autoscroll when new messages are added
if (div.offsetHeight + div.scrollTop > div.scrollHeight - 20) {
tick().then(() => {
div.scrollTo(0, div.scrollHeight);
});
}
});
</script>
<div bind:this={div}>
{#each messages as message}
<p>{message}</p>
{/each}
</div>
```
Apart from the timing, `$effect.pre` works exactly like `$effect`.
## `$effect.tracking`
The `$effect.tracking` rune is an advanced feature that tells you whether or not the code is running inside a tracking context, such as an effect or inside your template ([demo](/playground/untitled#H4sIAAAAAAAACn3PwYrCMBDG8VeZDYIt2PYeY8Dn2HrIhqkU08nQjItS-u6buAt7UDzmz8ePyaKGMWBS-nNRcmdU-hHUTpGbyuvI3KZvDFLal0v4qvtIgiSZUSb5eWSxPfWSc4oB2xDP1XYk8HHiSHkICeXKeruDDQ4Demlldv4y0rmq6z10HQwuJMxGVv4mVVXDwcJS0jP9u3knynwtoKz1vifT_Z9Jhm0WBCcOTlDD8kyspmML5qNpHg40jc3fFryJ0iWsp_UHgz3180oBAAA=)):
```svelte
<script>
console.log('in component setup:', $effect.tracking()); // false
$effect(() => {
console.log('in effect:', $effect.tracking()); // true
});
</script>
<p>in template: {$effect.tracking()}</p> <!-- true -->
```
It is used to implement abstractions like [`createSubscriber`](/docs/svelte/svelte-reactivity#createSubscriber), which will create listeners to update reactive values but _only_ if those values are being tracked (rather than, for example, read inside an event handler).
## `$effect.root`
The `$effect.root` rune is an advanced feature that creates a non-tracked scope that doesn't auto-cleanup. This is useful for nested effects that you want to manually control. This rune also allows for the creation of effects outside of the component initialisation phase.
```js
const destroy = $effect.root(() => {
$effect(() => {
// setup
});
return () => {
// cleanup
};
});
// later...
destroy();
```
## When not to use `$effect`
In general, `$effect` is best considered something of an escape hatch — useful for things like analytics and direct DOM manipulation — rather than a tool you should use frequently. In particular, avoid using it to synchronise state. Instead of this...
```svelte
<script>
let count = $state(0);
let doubled = $state();
// don't do this!
$effect(() => {
doubled = count * 2;
});
</script>
```
...do this:
```svelte
<script>
let count = $state(0);
let doubled = $derived(count * 2);
</script>
```
> [!NOTE] For things that are more complicated than a simple expression like `count * 2`, you can also use `$derived.by`.
If you're using an effect because you want to be able to reassign the derived value (to build an optimistic UI, for example) note that [deriveds can be directly overridden]($derived#Overriding-derived-values) as of Svelte 5.25.
You might be tempted to do something convoluted with effects to link one value to another. The following example shows two inputs for "money spent" and "money left" that are connected to each other. If you update one, the other should update accordingly. Don't use effects for this ([demo](/playground/untitled#H4sIAAAAAAAACpVRy26DMBD8FcvKgUhtoIdeHBwp31F6MGSJkBbHwksEQvx77aWQqooq9bgzOzP7mGTdIHipPiZJowOpGJAv0po2VmfnDv4OSBErjYdneHWzBJaCjcx91TWOToUtCIEE3cig0OIty44r5l1oDtjOkyFIsv3GINQ_CNYyGegd1DVUlCR7oU9iilDUcP8S8roYs9n8p2wdYNVFm4csTx872BxNCcjr5I11fdgonEkXsjP2CoUUZWMv6m6wBz2x7yxaM-iJvWeRsvSbSVeUy5i0uf8vKA78NIeJLSZWv1I8jQjLdyK4XuTSeIdmVKJGGI4LdjVOiezwDu1yG74My8PLCQaSiroe5s_5C2PHrkVGAgAA)):
```svelte
<script>
let total = 100;
let spent = $state(0);
let left = $state(total);
$effect(() => {
left = total - spent;
});
$effect(() => {
spent = total - left;
});
</script>
<label>
<input type="range" bind:value={spent} max={total} />
{spent}/{total} spent
</label>
<label>
<input type="range" bind:value={left} max={total} />
{left}/{total} left
</label>
```
Instead, use `oninput` callbacks or — better still — [function bindings](bind#Function-bindings) where possible ([demo](/playground/untitled#H4sIAAAAAAAAE51SsW6DMBT8FcvqABINdOhCIFKXTt06lg4GHpElYyz8iECIf69tcIIipo6-u3f3fPZMJWuBpvRzkBXyTpKSy5rLq6YRbbgATdOfmeKkrMgCBt9GPpQ66RsItFjJNBzhVScRJBobmumq5wovhSxQABLskAmSk7ckOXtMKyM22ItGhhAk4Z0R0OwIN-tIQzd-90HVhvy2HsGNiQFCMltBgd7XoecV2xzXNV7XaEcth7ZfRv7kujnsTX2Qd7USb5rFjwZkJlgJwpWRcakG04cpOS9oz-QVCuoeInXW-RyEJL-sG0b7Wy6kZWM-u7CFxM5tdrIl9qg72vB74H-y7T2iXROHyVb0CLanp1yNk4D1A1jQ91hzrQSbUtIIGLcir0ylJDm9Q7urz42bX4UwIk2xH2D5Xf4A7SeMcMQCAAA=)):
```svelte
<script>
let total = 100;
let spent = $state(0);
let left = $state(total);
function updateSpent(value) {
spent = value;
left = total - spent;
}
function updateLeft(value) {
left = value;
spent = total - left;
}
</script>
<label>
<input type="range" bind:value={() => spent, updateSpent} max={total} />
{spent}/{total} spent
</label>
<label>
<input type="range" bind:value={() => left, updateLeft} max={total} />
{left}/{total} left
</label>
```
If you absolutely have to update `$state` within an effect and run into an infinite loop because you read and write to the same `$state`, use [untrack](svelte#untrack).
# $props
The inputs to a component are referred to as _props_, which is short for _properties_. You pass props to components just like you pass attributes to elements:
```svelte
<!--- file: App.svelte --->
<script>
import MyComponent from './MyComponent.svelte';
</script>
<MyComponent adjective="cool" />
```
On the other side, inside `MyComponent.svelte`, we can receive props with the `$props` rune...
```svelte
<!--- file: MyComponent.svelte --->
<script>
let props = $props();
</script>
<p>this component is {props.adjective}</p>
```
...though more commonly, you'll [_destructure_](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment) your props:
```svelte
<!--- file: MyComponent.svelte --->
<script>
let +++{ adjective }+++ = $props();
</script>
<p>this component is {+++adjective+++}</p>
```
## Fallback values
Destructuring allows us to declare fallback values, which are used if the parent component does not set a given prop (or the value is `undefined`):
```js
let { adjective = 'happy' } = $props();
```
> [!NOTE] Fallback values are not turned into reactive state proxies (see [Updating props](#Updating-props) for more info)
## Renaming props
We can also use the destructuring assignment to rename props, which is necessary if they're invalid identifiers, or a JavaScript keyword like `super`:
```js
let { super: trouper = 'lights are gonna find me' } = $props();
```
## Rest props
Finally, we can use a _rest property_ to get, well, the rest of the props:
```js
let { a, b, c, ...others } = $props();
```
## Updating props
References to a prop inside a component update when the prop itself updates — when `count` changes in `App.svelte`, it will also change inside `Child.svelte`. But the child component is able to temporarily override the prop value, which can be useful for unsaved ephemeral state ([demo](/playground/untitled#H4sIAAAAAAAAE6WQ0WrDMAxFf0WIQR0Wmu3VTQJln7HsIfVcZubIxlbGRvC_DzuBraN92qPula50tODZWB1RPi_IX16jLALWSOOUq6P3-_ihLWftNEZ9TVeOWBNHlNhGFYznfqCBzeRdYHh6M_YVzsFNsNs3pdpGd4eBcqPVDMrNxNDBXeSRtXioDgO1zU8ataeZ2RE4Utao924RFXQ9iHXwvoPHKpW1xY4g_Bg0cSVhKS0p560Za95612ZC02ONrD8ZJYdZp_rGQ37ff_mSP86Np2TWZaNNmdcH56P4P67K66_SXoK9pG-5dF5Z9QEAAA==)):