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Svelte

Before we begin

This page contains detailed API reference documentation. It's intended to be a resource for people who already have some familiarity with Svelte.

If that's not you (yet), you may prefer to visit the interactive tutorial or the examples before consulting this reference.

Don't be shy about asking for help in the Discord chatroom.

Using an older version of Svelte? Have a look at the v2 docs.

Getting started

To try Svelte in an interactive online environment you can try the REPL or StackBlitz.

To create a project locally we recommend using SvelteKit, the official application framework from the Svelte team:

npm create svelte@latest myapp
cd myapp
npm install
npm run dev

SvelteKit will handle calling the Svelte compiler to convert your .svelte files into .js files that create the DOM and .css files that style it. It also provides all the other pieces you need to build a web application such as a development server, routing, and deployment. SvelteKit utilizes Vite to build your code and handle server-side rendering (SSR). There are plugins for all the major web bundlers to handle Svelte compilation, which will output .js and .css that you can insert into your HTML, but most others won't handle SSR.

If you don't need a full-fledged app framework and instead want to build a simple frontend-only site/app, you can also use Svelte (without Kit) with Vite by running npm init vite and selecting the svelte option. With this, npm run build will generate HTML, JS and CSS files inside the dist directory.

The Svelte team maintains a VS Code extension and there are integrations with various other editors and tools as well.

If you're having trouble, get help on Discord or StackOverflow.

Component format

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.

<script>
	// logic goes here
</script>

<!-- markup (zero or more items) goes here -->

<style>
	/* styles go here */
</style>

<script>

A <script> block contains JavaScript that runs when a component instance is created. Variables declared (or imported) at the top level are 'visible' from the component's markup. There are four additional rules:

1. `export` creates a component prop

Svelte uses the export keyword to mark a variable declaration as a property or prop, which means it becomes accessible to consumers of the component (see the section on attributes and props for more information).

<script>
	export let foo;

	// Values that are passed in as props
	// are immediately available
	console.log({ foo });
</script>

You can specify a default initial value for a prop. It will be used if the component's consumer doesn't specify the prop on the component (or if its initial value is undefined) when instantiating the component. Note that whenever a prop is removed by the consumer, its value is set to undefined rather than the initial value.

In development mode (see the compiler options), a warning will be printed if no default initial value is provided and the consumer does not specify a value. To squelch this warning, ensure that a default initial value is specified, even if it is undefined.

<script>
	export let bar = 'optional default initial value';
	export let baz = undefined;
</script>

If you export a const, class or function, it is readonly from outside the component. Functions are valid prop values, however, as shown below.

<script>
	// these are readonly
	export const thisIs = 'readonly';

	export function greet(name) {
		alert(`hello ${name}!`);
	}

	// this is a prop
	export let format = n => n.toFixed(2);
</script>

Readonly props can be accessed as properties on the element, tied to the component using bind:this syntax.

You can use reserved words as prop names.

<script>
	let className;

	// creates a `class` property, even
	// though it is a reserved word
	export { className as class };
</script>

2. Assignments are 'reactive'

To change component state and trigger a re-render, just assign to a locally declared variable.

Update expressions (count += 1) and property assignments (obj.x = y) have the same effect.

<script>
	let count = 0;

	function handleClick () {
		// calling this function will trigger an
		// update if the markup references `count`
		count = count + 1;
	}
</script>

Because Svelte's reactivity is based on assignments, using array methods like .push() and .splice() won't automatically trigger updates. A subsequent assignment is required to trigger the update. This and more details can also be found in the tutorial.

<script>
	let arr = [0, 1];

	function handleClick () {
		// this method call does not trigger an update
		arr.push(2);
		// this assignment will trigger an update
		// if the markup references `arr`
		arr = arr
	}
</script>

Svelte's <script> blocks are run only when the component is created, so assignments within a <script> block are not automatically run again when a prop updates. If you'd like to track changes to a prop, see the next example in the following section.

<script>
	export let person;
	// this will only set `name` on component creation
	// it will not update when `person` does
	let { name } = person;
</script>

3. `$:` marks a statement as reactive

Any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with the $: JS label syntax. Reactive statements run after other script code and before the component markup is rendered, whenever the values that they depend on have changed.

<script>
	export let title;
	export let person

	// this will update `document.title` whenever
	// the `title` prop changes
	$: document.title = title;

	$: {
		console.log(`multiple statements can be combined`);
		console.log(`the current title is ${title}`);
	}

	// this will update `name` when 'person' changes
	$: ({ name } = person);

	// don't do this. it will run before the previous line
	let name2 = name;
</script>

Only values which directly appear within the $: block will become dependencies of the reactive statement. For example, in the code below total will only update when x changes, but not y.

<script>
	let x = 0;
	let y = 0;
	
	function yPlusAValue(value) {
		return value + y;
	}
	
	$: total = yPlusAValue(x);
</script>

Total: {total}
<button on:click={() => x++}>
	Increment X
</button>

<button on:click={() => y++}>
	Increment Y
</button>

It is important to note that the reactive blocks are ordered via simple static analysis at compile time, and all the compiler looks at are the variables that are assigned to and used within the block itself, not in any functions called by them. This means that yDependent will not be updated when x is updated in the following example:

<script>
	let x = 0;
	let y = 0;
	
	const setY = (value) => {
		y = value;
	}
	
	$: yDependent = y;
	$: setY(x);
</script>

Moving the line $: yDependent = y below $: setY(x) will cause yDependent to be updated when x is updated.

If a statement consists entirely of an assignment to an undeclared variable, Svelte will inject a let declaration on your behalf.

<script>
	export let num;

	// we don't need to declare `squared` and `cubed`
	// — Svelte does it for us
	$: squared = num * num;
	$: cubed = squared * num;
</script>

4. Prefix stores with `$` to access their values

A store is an object that allows reactive access to a value via a simple store contract. The svelte/store module contains minimal store implementations which fulfil this contract.

Any time you have a reference to a store, you can access its value inside a component by prefixing it with the $ character. This causes Svelte to declare the prefixed variable, subscribe to the store at component initialization and unsubscribe when appropriate.

Assignments to $-prefixed variables require that the variable be a writable store, and will result in a call to the store's .set method.

Note that the store must be declared at the top level of the component — not inside an if block or a function, for example.

Local variables (that do not represent store values) must not have a $ prefix.

<script>
	import { writable } from 'svelte/store';

	const count = writable(0);
	console.log($count); // logs 0

	count.set(1);
	console.log($count); // logs 1

	$count = 2;
	console.log($count); // logs 2
</script>

Store contract

store = { subscribe: (subscription: (value: any) => void) => (() => void), set?: (value: any) => void }

You can create your own stores without relying on svelte/store, by implementing the store contract:

A store must contain a .subscribe method, which must accept as its argument a subscription function. This subscription function must be immediately and synchronously called with the store's current value upon calling .subscribe. All of a store's active subscription functions must later be synchronously called whenever the store's value changes.
The .subscribe method must return an unsubscribe function. Calling an unsubscribe function must stop its subscription, and its corresponding subscription function must not be called again by the store.
A store may optionally contain a .set method, which must accept as its argument a new value for the store, and which synchronously calls all of the store's active subscription functions. Such a store is called a writable store.

For interoperability with RxJS Observables, the .subscribe method is also allowed to return an object with an .unsubscribe method, rather than return the unsubscription function directly. Note however that unless .subscribe synchronously calls the subscription (which is not required by the Observable spec), Svelte will see the value of the store as undefined until it does.

<script context="module">

A <script> tag with a context="module" attribute runs once when the module first evaluates, rather than for each component instance. Values declared in this block are accessible from a regular <script> (and the component markup) but not vice versa.

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.

Variables defined in module scripts are not reactive — reassigning them will not trigger a rerender even though the variable itself will update. For values shared between multiple components, consider using a store.

<script context="module">
	let totalComponents = 0;

	// this allows an importer to do e.g.
	// `import Example, { alertTotal } from './Example.svelte'`
	export function alertTotal() {
		alert(totalComponents);
	}
</script>

<script>
	totalComponents += 1;
	console.log(`total number of times this component has been created: ${totalComponents}`);
</script>

<style>

CSS inside a <style> block will be scoped to that component.

This works by adding a class to affected elements, which is based on a hash of the component styles (e.g. svelte-123xyz).

<style>
	p {
		/* this will only affect <p> elements in this component */
		color: burlywood;
	}
</style>

To apply styles to a selector globally, use the :global(...) modifier.

<style>
	:global(body) {
		/* this will apply to <body> */
		margin: 0;
	}

	div :global(strong) {
		/* this will apply to all <strong> elements, in any
			 component, that are inside <div> elements belonging
			 to this component */
		color: goldenrod;
	}

	p:global(.red) {
		/* this will apply to all <p> elements belonging to this 
			 component with a class of red, even if class="red" does
			 not initially appear in the markup, and is instead 
			 added at runtime. This is useful when the class 
			 of the element is dynamically applied, for instance 
			 when updating the element's classList property directly. */
	}
</style>

If you want to make @keyframes that are accessible globally, you need to prepend your keyframe names with -global-.

The -global- part will be removed when compiled, and the keyframe then be referenced using just my-animation-name elsewhere in your code.

<style>
	@keyframes -global-my-animation-name {...}
</style>

There should only be 1 top-level <style> tag per component.

However, it is possible to have <style> tag nested inside other elements or logic blocks.

In that case, the <style> tag will be inserted as-is into the DOM, no scoping or processing will be done on the <style> tag.

<div>
	<style>
		/* this style tag will be inserted as-is */
		div {
			/* this will apply to all `<div>` elements in the DOM */
			color: red;
		}
	</style>
</div>

Template syntax

Attributes and props

By default, attributes work exactly like their HTML counterparts.

<div class="foo">
	<button disabled>can't touch this</button>
</div>

As in HTML, values may be unquoted.

<input type=checkbox>

Attribute values can contain JavaScript expressions.

<a href="page/{p}">page {p}</a>

Or they can be JavaScript expressions.

<button disabled={!clickable}>...</button>

Boolean attributes are included on the element if their value is truthy and excluded if it's falsy.

All other attributes are included unless their value is nullish (null or undefined).

<input required={false} placeholder="This input field is not required">
<div title={null}>This div has no title attribute</div>

An expression might include characters that would cause syntax highlighting to fail in regular HTML, so quoting the value is permitted. The quotes do not affect how the value is parsed:

<button disabled="{number !== 42}">...</button>

When the attribute name and value match (name={name}), they can be replaced with {name}.

<!-- These are equivalent -->
<button disabled={disabled}>...</button>
<button {disabled}>...</button>

By convention, values passed to components are referred to as properties or props rather than attributes, which are a feature of the DOM.

As with elements, name={name} can be replaced with the {name} shorthand.

<Widget foo={bar} answer={42} text="hello"/>

Spread attributes allow many attributes or properties to be passed to an element or component at once.

An element or component can have multiple spread attributes, interspersed with regular ones.

<Widget {...things}/>

$$props references all props that are passed to a component, including ones that are not declared with export. It is not generally recommended, as it is difficult for Svelte to optimise. But it can be useful in rare cases – for example, when you don't know at compile time what props might be passed to a component.

<Widget {...$$props}/>

$$restProps contains only the props which are not declared with export. It can be used to pass down other unknown attributes to an element in a component. It shares the same optimisation problems as $$props, and is likewise not recommended.

<input {...$$restProps}>

The value attribute of an input element or its children option elements must not be set with spread attributes when using bind:group or bind:checked. Svelte needs to be able to see the element's value directly in the markup in these cases so that it can link it to the bound variable.

Sometimes, the attribute order matters as Svelte sets attributes sequentially in JavaScript. For example, <input type="range" min="0" max="1" value={0.5} step="0.1"/>, Svelte will attempt to set the value to 1 (rounding up from 0.5 as the step by default is 1), and then set the step to 0.1. To fix this, change it to <input type="range" min="0" max="1" step="0.1" value={0.5}/>.

Another example is <img src="..." loading="lazy" />. Svelte will set the img src before making the img element loading="lazy", which is probably too late. Change this to <img loading="lazy" src="..."> to make the image lazily loaded.

Text expressions

{expression}

Text can also contain JavaScript expressions:

If you're using a regular expression (RegExp) literal notation, you'll need to wrap it in parentheses.

<h1>Hello {name}!</h1>
<p>{a} + {b} = {a + b}.</p>

<div>{(/^[A-Za-z ]+$/).test(value) ? x : y}</div>

Comments

You can use HTML comments inside components.

<!-- this is a comment! -->
<h1>Hello world</h1>

Comments beginning with svelte-ignore disable warnings for the next block of markup. Usually, these are accessibility warnings; make sure that you're disabling them for a good reason.

<!-- svelte-ignore a11y-autofocus -->
<input bind:value={name} autofocus>

{#if ...}

{#if expression}...{/if}

{#if expression}...{:else if expression}...{/if}

{#if expression}...{:else}...{/if}

Content that is conditionally rendered can be wrapped in an if block.

{#if answer === 42}
	<p>what was the question?</p>
{/if}

Additional conditions can be added with {:else if expression}, optionally ending in an {:else} clause.

{#if porridge.temperature > 100}
	<p>too hot!</p>
{:else if 80 > porridge.temperature}
	<p>too cold!</p>
{:else}
	<p>just right!</p>
{/if}

{#each ...}

{#each expression as name}...{/each}

{#each expression as name, index}...{/each}

{#each expression as name (key)}...{/each}

{#each expression as name, index (key)}...{/each}

{#each expression as name}...{:else}...{/each}

Iterating over lists of values can be done with an each block.

<h1>Shopping list</h1>
<ul>
	{#each items as item}
		<li>{item.name} x {item.qty}</li>
	{/each}
</ul>

You can use each blocks to iterate over any array or array-like value — that is, any object with a length property.

An each block can also specify an index, equivalent to the second argument in an array.map(...) callback:

{#each items as item, i}
	<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}

If a key expression is provided — which must uniquely identify each list item — Svelte will use it to diff the list when data changes, rather than adding or removing items at the end. The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.

{#each items as item (item.id)}
	<li>{item.name} x {item.qty}</li>
{/each}

<!-- or with additional index value -->
{#each items as item, i (item.id)}
	<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}

You can freely use destructuring and rest patterns in each blocks.

{#each items as { id, name, qty }, i (id)}
	<li>{i + 1}: {name} x {qty}</li>
{/each}

{#each objects as { id, ...rest }}
	<li><span>{id}</span><MyComponent {...rest}/></li>
{/each}

{#each items as [id, ...rest]}
	<li><span>{id}</span><MyComponent values={rest}/></li>
{/each}

An each block can also have an {:else} clause, which is rendered if the list is empty.

{#each todos as todo}
	<p>{todo.text}</p>
{:else}
	<p>No tasks today!</p>
{/each}

{#await ...}

{#await expression}...{:then name}...{:catch name}...{/await}

{#await expression}...{:then name}...{/await}

{#await expression then name}...{/await}

{#await expression catch name}...{/await}

Await blocks allow you to branch on the three possible states of a Promise — pending, fulfilled or rejected. In SSR mode, only the pending state will be rendered on the server.

{#await promise}
	<!-- promise is pending -->
	<p>waiting for the promise to resolve...</p>
{:then value}
	<!-- promise was fulfilled -->
	<p>The value is {value}</p>
{:catch error}
	<!-- promise was rejected -->
	<p>Something went wrong: {error.message}</p>
{/await}

The catch block can be omitted if you don't need to render anything when the promise rejects (or no error is possible).

{#await promise}
	<!-- promise is pending -->
	<p>waiting for the promise to resolve...</p>
{:then value}
	<!-- promise was fulfilled -->
	<p>The value is {value}</p>
{/await}

If you don't care about the pending state, you can also omit the initial block.

{#await promise then value}
	<p>The value is {value}</p>
{/await}

Similarly, if you only want to show the error state, you can omit the then block.

{#await promise catch error}
	<p>The error is {error}</p>
{/await}

{#key ...}

{#key expression}...{/key}

Key blocks destroy and recreate their contents when the value of an expression changes.

This is useful if you want an element to play its transition whenever a value changes.

{#key value}
	<div transition:fade>{value}</div>
{/key}

When used around components, this will cause them to be reinstantiated and reinitialised.

{#key value}
	<Component />
{/key}

{@html ...}

{@html expression}

In a text expression, characters like < and > are escaped; however, with HTML expressions, they're not.

The expression should be valid standalone HTML — {@html "<div>"}content{@html "</div>"} will not work, because </div> is not valid HTML. It also will not compile Svelte code.

Svelte does not sanitize expressions before injecting HTML. If the data comes from an untrusted source, you must sanitize it, or you are exposing your users to an XSS vulnerability.

<div class="blog-post">
	<h1>{post.title}</h1>
	{@html post.content}
</div>

{@debug ...}

{@debug}

{@debug var1, var2, ..., varN}

The {@debug ...} tag offers an alternative to console.log(...). It logs the values of specific variables whenever they change, and pauses code execution if you have devtools open.

<script>
	let user = {
		firstname: 'Ada',
		lastname: 'Lovelace'
	};
</script>

{@debug user}

<h1>Hello {user.firstname}!</h1>

{@debug ...} accepts a comma-separated list of variable names (not arbitrary expressions).

<!-- Compiles -->
{@debug user}
{@debug user1, user2, user3}

<!-- WON'T compile -->
{@debug user.firstname}
{@debug myArray[0]}
{@debug !isReady}
{@debug typeof user === 'object'}

The {@debug} tag without any arguments will insert a debugger statement that gets triggered when any state changes, as opposed to the specified variables.

{@const ...}

{@const assignment}

The {@const ...} tag defines a local constant.

<script>
	export let boxes;
</script>

{#each boxes as box}
	{@const area = box.width * box.height}
	{box.width} * {box.height} = {area}
{/each}

{@const} is only allowed as direct child of {#if}, {:else if}, {:else}, {#each}, {:then}, {:catch}, <Component /> or <svelte:fragment />.

Element directives

As well as attributes, elements can have directives, which control the element's behaviour in some way.

on:eventname

on:eventname={handler}

on:eventname|modifiers={handler}

Use the on: directive to listen to DOM events.

<script>
	let count = 0;

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

<button on:click={handleClick}>
	count: {count}
</button>

Handlers can be declared inline with no performance penalty. As with attributes, directive values may be quoted for the sake of syntax highlighters.

<button on:click="{() => count += 1}">
	count: {count}
</button>

Add modifiers to DOM events with the | character.

<form on:submit|preventDefault={handleSubmit}>
	<!-- the `submit` event's default is prevented,
	     so the page won't reload -->
</form>

The following modifiers are available:

preventDefault — calls event.preventDefault() before running the handler
stopPropagation — calls event.stopPropagation(), preventing the event reaching the next element
passive — improves scrolling performance on touch/wheel events (Svelte will add it automatically where it's safe to do so)
nonpassive — explicitly set passive: false
capture — fires the handler during the capture phase instead of the bubbling phase
once — remove the handler after the first time it runs
self — only trigger handler if event.target is the element itself
trusted — only trigger handler if event.isTrusted is true. I.e. if the event is triggered by a user action.

Modifiers can be chained together, e.g. on:click|once|capture={...}.

If the on: directive is used without a value, the component will forward the event, meaning that a consumer of the component can listen for it.

<button on:click>
	The component itself will emit the click event
</button>

It's possible to have multiple event listeners for the same event:

<script>
	let counter = 0;
	function increment() {
		counter = counter + 1;
	}

	function track(event) {
		trackEvent(event)
	}
</script>

<button on:click={increment} on:click={track}>Click me!</button>

bind:property

bind:property={variable}

Data ordinarily flows down, from parent to child. The bind: directive allows data to flow the other way, from child to parent. Most bindings are specific to particular elements.

The simplest bindings reflect the value of a property, such as input.value.

<input bind:value={name}>
<textarea bind:value={text}></textarea>

<input type="checkbox" bind:checked={yes}>

If the name matches the value, you can use shorthand.

<!-- These are equivalent -->
<input bind:value={value}>
<input bind:value>

Numeric input values are coerced; even though input.value is a string as far as the DOM is concerned, Svelte will treat it as a number. If the input is empty or invalid (in the case of type="number"), the value is undefined.

<input type="number" bind:value={num}>
<input type="range" bind:value={num}>

On <input> elements with type="file", you can use bind:files to get the FileList of selected files. It is readonly.

<label for="avatar">Upload a picture:</label>
<input
	accept="image/png, image/jpeg"
	bind:files
	id="avatar"
	name="avatar"
	type="file"
/>

If you're using bind: directives together with on: directives, the order that they're defined in affects the value of the bound variable when the event handler is called.

<script>
	let value = 'Hello World';
</script>

<input
	on:input="{() => console.log('Old value:', value)}"
	bind:value
	on:input="{() => console.log('New value:', value)}"
/>

Here we were binding to the value of a text input, which uses the input event. Bindings on other elements may use different events such as change.

Binding `<select>` value

A <select> value binding corresponds to the value property on the selected <option>, which can be any value (not just strings, as is normally the case in the DOM).

<select bind:value={selected}>
	<option value={a}>a</option>
	<option value={b}>b</option>
	<option value={c}>c</option>
</select>

A <select multiple> element behaves similarly to a checkbox group.

<select multiple bind:value={fillings}>
	<option value="Rice">Rice</option>
	<option value="Beans">Beans</option>
	<option value="Cheese">Cheese</option>
	<option value="Guac (extra)">Guac (extra)</option>
</select>

When the value of an <option> matches its text content, the attribute can be omitted.

<select multiple bind:value={fillings}>
	<option>Rice</option>
	<option>Beans</option>
	<option>Cheese</option>
	<option>Guac (extra)</option>
</select>

Elements with the contenteditable attribute support innerHTML and textContent bindings.

<div contenteditable="true" bind:innerHTML={html}></div>

<details> elements support binding to the open property.

<details bind:open={isOpen}>
	<summary>Details</summary>
	<p>
		Something small enough to escape casual notice.
	</p>
</details>

Media element bindings

Media elements (<audio> and <video>) have their own set of bindings — six readonly ones...

duration (readonly) — the total duration of the video, in seconds
buffered (readonly) — an array of {start, end} objects
played (readonly) — ditto
seekable (readonly) — ditto
seeking (readonly) — boolean
ended (readonly) — boolean

...and five two-way bindings:

currentTime — the current playback time in the video, in seconds
playbackRate — how fast or slow to play the video, where 1 is 'normal'
paused — this one should be self-explanatory
volume — a value between 0 and 1
muted — a boolean value indicating whether the player is muted

Videos additionally have readonly videoWidth and videoHeight bindings.

<video
	src={clip}
	bind:duration
	bind:buffered
	bind:played
	bind:seekable
	bind:seeking
	bind:ended
	bind:currentTime
	bind:playbackRate
	bind:paused
	bind:volume
	bind:muted
	bind:videoWidth
	bind:videoHeight
></video>

Block-level element bindings

Block-level elements have 4 read-only bindings, measured using a technique similar to this one:

clientWidth
clientHeight
offsetWidth
offsetHeight

<div
	bind:offsetWidth={width}
	bind:offsetHeight={height}
>
	<Chart {width} {height}/>
</div>

bind:group

bind:group={variable}

Inputs that work together can use bind:group.

<script>
	let tortilla = 'Plain';
	let fillings = [];
</script>

<!-- grouped radio inputs are mutually exclusive -->
<input type="radio" bind:group={tortilla} value="Plain">
<input type="radio" bind:group={tortilla} value="Whole wheat">
<input type="radio" bind:group={tortilla} value="Spinach">

<!-- grouped checkbox inputs populate an array -->
<input type="checkbox" bind:group={fillings} value="Rice">
<input type="checkbox" bind:group={fillings} value="Beans">
<input type="checkbox" bind:group={fillings} value="Cheese">
<input type="checkbox" bind:group={fillings} value="Guac (extra)">

bind:this

bind:this={dom_node}

To get a reference to a DOM node, use bind:this.

<script>
	import { onMount } from 'svelte';

	let canvasElement;

	onMount(() => {
		const ctx = canvasElement.getContext('2d');
		drawStuff(ctx);
	});
</script>

<canvas bind:this={canvasElement}></canvas>

class:name

class:name={value}

class:name

A class: directive provides a shorter way of toggling a class on an element.

<!-- These are equivalent -->
<div class="{active ? 'active' : ''}">...</div>
<div class:active={active}>...</div>

<!-- Shorthand, for when name and value match -->
<div class:active>...</div>

<!-- Multiple class toggles can be included -->
<div class:active class:inactive={!active} class:isAdmin>...</div>

style:property

style:property={value}

style:property="value"

style:property

The style: directive provides a shorthand for setting multiple styles on an element.

<!-- These are equivalent -->
<div style:color="red">...</div>
<div style="color: red;">...</div>

<!-- Variables can be used -->
<div style:color={myColor}>...</div>

<!-- Shorthand, for when property and variable name match -->
<div style:color>...</div>

<!-- Multiple styles can be included -->
<div style:color style:width="12rem" style:background-color={darkMode ? "black" : "white"}>...</div>

<!-- Styles can be marked as important -->
<div style:color|important="red">...</div>

When style: directives are combined with style attributes, the directives will take precedence:

<div style="color: blue;" style:color="red">This will be red</div>

use:action

use:action

use:action={parameters}

action = (node: HTMLElement, parameters: any) => {
	update?: (parameters: any) => void,
	destroy?: () => void
}

Actions are functions that are called when an element is created. They can return an object with a destroy method that is called after the element is unmounted:

<script>
	function foo(node) {
		// the node has been mounted in the DOM

		return {
			destroy() {
				// the node has been removed from the DOM
			}
		};
	}
</script>

<div use:foo></div>

An action can have a parameter. If the returned value has an update method, it will be called whenever that parameter changes, immediately after Svelte has applied updates to the markup.

Don't worry about the fact that we're redeclaring the foo function for every component instance — Svelte will hoist any functions that don't depend on local state out of the component definition.

<script>
	export let bar;

	function foo(node, bar) {
		// the node has been mounted in the DOM

		return {
			update(bar) {
				// the value of `bar` has changed
			},

			destroy() {
				// the node has been removed from the DOM
			}
		};
	}
</script>

<div use:foo={bar}></div>

transition:fn

transition:fn

transition:fn={params}

transition:fn|local

transition:fn|local={params}

transition = (node: HTMLElement, params: any, options: { direction: 'in' | 'out' | 'both' }) => {
	delay?: number,
	duration?: number,
	easing?: (t: number) => number,
	css?: (t: number, u: number) => string,
	tick?: (t: number, u: number) => void
}

A transition is triggered by an element entering or leaving the DOM as a result of a state change.

When a block is transitioning out, all elements inside the block, including those that do not have their own transitions, are kept in the DOM until every transition in the block has been completed.

The transition: directive indicates a bidirectional transition, which means it can be smoothly reversed while the transition is in progress.

{#if visible}
	<div transition:fade>
		fades in and out
	</div>
{/if}

By default intro transitions will not play on first render. You can modify this behaviour by setting intro: true when you create a component.

Transition parameters

Like actions, transitions can have parameters.

(The double {{curlies}} aren't a special syntax; this is an object literal inside an expression tag.)

{#if visible}
	<div transition:fade="{{ duration: 2000 }}">
		fades in and out over two seconds
	</div>
{/if}

Custom transition functions

Transitions can use custom functions. If the returned object has a css function, Svelte will create a CSS animation that plays on the element.

The t argument passed to css is a value between 0 and 1 after the easing function has been applied. In transitions run from 0 to 1, out transitions run from 1 to 0 — in other words 1 is the element's natural state, as though no transition had been applied. The u argument is equal to 1 - t.

The function is called repeatedly before the transition begins, with different t and u arguments.

<script>
	import { elasticOut } from 'svelte/easing';

	export let visible;

	function whoosh(node, params) {
		const existingTransform = getComputedStyle(node).transform.replace('none', '');

		return {
			delay: params.delay || 0,
			duration: params.duration || 400,
			easing: params.easing || elasticOut,
			css: (t, u) => `transform: ${existingTransform} scale(${t})`
		};
	}
</script>

{#if visible}
	<div in:whoosh>
		whooshes in
	</div>
{/if}

A custom transition function can also return a tick function, which is called during the transition with the same t and u arguments.

If it's possible to use css instead of tick, do so — CSS animations can run off the main thread, preventing jank on slower devices.

<script>
	export let visible = false;

	function typewriter(node, { speed = 1 }) {
		const valid = (
			node.childNodes.length === 1 &&
			node.childNodes[0].nodeType === Node.TEXT_NODE
		);

		if (!valid) {
			throw new Error(`This transition only works on elements with a single text node child`);
		}

		const text = node.textContent;
		const duration = text.length / (speed * 0.01);

		return {
			duration,
			tick: t => {
				const i = ~~(text.length * t);
				node.textContent = text.slice(0, i);
			}
		};
	}
</script>

{#if visible}
	<p in:typewriter="{{ speed: 1 }}">
		The quick brown fox jumps over the lazy dog
	</p>
{/if}

If a transition returns a function instead of a transition object, the function will be called in the next microtask. This allows multiple transitions to coordinate, making crossfade effects possible.

Transition functions also receive a third argument, options, which contains information about the transition.

Available values in the options object are:

direction - one of in, out, or both depending on the type of transition

Transition events

An element with transitions will dispatch the following events in addition to any standard DOM events:

introstart
introend
outrostart
outroend

{#if visible}
	<p
		transition:fly="{{ y: 200, duration: 2000 }}"
		on:introstart="{() => status = 'intro started'}"
		on:outrostart="{() => status = 'outro started'}"
		on:introend="{() => status = 'intro ended'}"
		on:outroend="{() => status = 'outro ended'}"
	>
		Flies in and out
	</p>
{/if}

Local transitions only play when the block they belong to is created or destroyed, not when parent blocks are created or destroyed.

{#if x}
	{#if y}
		<p transition:fade>
			fades in and out when x or y change
		</p>

		<p transition:fade|local>
			fades in and out only when y changes
		</p>
	{/if}
{/if}

in:fn/out:fn

in:fn

in:fn={params}

in:fn|local

in:fn|local={params}

out:fn

out:fn={params}

out:fn|local

out:fn|local={params}

Similar to transition:, but only applies to elements entering (in:) or leaving (out:) the DOM.

Unlike with transition:, transitions applied with in: and out: are not bidirectional — an in transition will continue to 'play' alongside the out transition, rather than reversing, if the block is outroed while the transition is in progress. If an out transition is aborted, transitions will restart from scratch.

{#if visible}
	<div in:fly out:fade>
		flies in, fades out
	</div>
{/if}

animate:fn

animate:name

animate:name={params}

animation = (node: HTMLElement, { from: DOMRect, to: DOMRect } , params: any) => {
	delay?: number,
	duration?: number,
	easing?: (t: number) => number,
	css?: (t: number, u: number) => string,
	tick?: (t: number, u: number) => void
}

DOMRect {
	bottom: number,
	height: number,
	left: number,
	right: number,
	top: number,
	width: number,
	x: number,
	y: number
}

An animation is triggered when the contents of a keyed each block are re-ordered. Animations do not run when an element is added or removed, only when the index of an existing data item within the each block changes. Animate directives must be on an element that is an immediate child of a keyed each block.

Animations can be used with Svelte's built-in animation functions or custom animation functions.

<!-- When `list` is reordered the animation will run-->
{#each list as item, index (item)}
	<li animate:flip>{item}</li>
{/each}

Animation Parameters

As with actions and transitions, animations can have parameters.

(The double {{curlies}} aren't a special syntax; this is an object literal inside an expression tag.)

{#each list as item, index (item)}
	<li animate:flip="{{ delay: 500 }}">{item}</li>
{/each}

Custom animation functions

Animations can use custom functions that provide the node, an animation object and any parameters as arguments. The animation parameter is an object containing from and to properties each containing a DOMRect describing the geometry of the element in its start and end positions. The from property is the DOMRect of the element in its starting position, and the to property is the DOMRect of the element in its final position after the list has been reordered and the DOM updated.

If the returned object has a css method, Svelte will create a CSS animation that plays on the element.

The t argument passed to css is a value that goes from 0 and 1 after the easing function has been applied. The u argument is equal to 1 - t.

The function is called repeatedly before the animation begins, with different t and u arguments.

<script>
	import { cubicOut } from 'svelte/easing';

	function whizz(node, { from, to }, params) {

		const dx = from.left - to.left;
		const dy = from.top - to.top;

		const d = Math.sqrt(dx * dx + dy * dy);

		return {
			delay: 0,
			duration: Math.sqrt(d) * 120,
			easing: cubicOut,
			css: (t, u) =>
				`transform: translate(${u * dx}px, ${u * dy}px) rotate(${t*360}deg);`
		};
	}
</script>

{#each list as item, index (item)}
	<div animate:whizz>{item}</div>
{/each}

A custom animation function can also return a tick function, which is called during the animation with the same t and u arguments.

If it's possible to use css instead of tick, do so — CSS animations can run off the main thread, preventing jank on slower devices.

<script>
	import { cubicOut } from 'svelte/easing';

	function whizz(node, { from, to }, params) {

		const dx = from.left - to.left;
		const dy = from.top - to.top;

		const d = Math.sqrt(dx * dx + dy * dy);

		return {
		delay: 0,
		duration: Math.sqrt(d) * 120,
		easing: cubicOut,
		tick: (t, u) =>
			Object.assign(node.style, {
				color: t > 0.5 ? 'Pink' : 'Blue'
			});
	};
	}
</script>

{#each list as item, index (item)}
	<div animate:whizz>{item}</div>
{/each}

Component directives

on:eventname

on:eventname={handler}

Components can emit events using createEventDispatcher, or by forwarding DOM events. Listening for component events looks the same as listening for DOM events:

<SomeComponent on:whatever={handler}/>

As with DOM events, if the on: directive is used without a value, the component will forward the event, meaning that a consumer of the component can listen for it.

<SomeComponent on:whatever/>

--style-props

--style-props="anycssvalue"

You can also pass styles as props to components for the purposes of theming, using CSS custom properties.

Svelte's implementation is essentially syntactic sugar for adding a wrapper element. This example:

<Slider
  bind:value
  min={0}
  --rail-color="black"
  --track-color="rgb(0, 0, 255)"
/>

Desugars to this:

<div style="display: contents; --rail-color: black; --track-color: rgb(0, 0, 255)">
  <Slider
    bind:value
    min={0}
    max={100}
  />
</div>

Note: Since this is an extra <div>, beware that your CSS structure might accidentally target this. Be mindful of this added wrapper element when using this feature.

For SVG namespace, the example above desugars into using <g> instead:

<g style="--rail-color: black; --track-color: rgb(0, 0, 255)">
  <Slider
    bind:value
    min={0}
    max={100}
  />
</g>

Note: Since this is an extra <g>, beware that your CSS structure might accidentally target this. Be mindful of this added wrapper element when using this feature.

Svelte's CSS Variables support allows for easily themeable components:

<!-- Slider.svelte -->
<style>
  .potato-slider-rail {
    background-color: var(--rail-color, var(--theme-color, 'purple'));
  }
</style>

So you can set a high level theme color:

/* global.css */
html {
  --theme-color: black;
}

Or override it at the consumer level:

<Slider --rail-color="goldenrod"/>

bind:property

bind:property={variable}

You can bind to component props using the same syntax as for elements.

<Keypad bind:value={pin}/>

bind:this

bind:this={component_instance}

Components also support bind:this, allowing you to interact with component instances programmatically.

Note that we can't do {cart.empty} since cart is undefined when the button is first rendered and throws an error.

<ShoppingCart bind:this={cart}/>

<button on:click={() => cart.empty()}>
	Empty shopping cart
</button>

`<slot>`

<slot><!-- optional fallback --></slot>

<slot name="x"><!-- optional fallback --></slot>

<slot prop={value}></slot>

Components can have child content, in the same way that elements can.

The content is exposed in the child component using the <slot> element, which can contain fallback content that is rendered if no children are provided.

<!-- Widget.svelte -->
<div>
	<slot>
		this fallback content will be rendered when no content is provided, like in the first example
	</slot>
</div>

<!-- App.svelte -->
<Widget></Widget> <!-- this component will render the default content -->

<Widget>
	<p>this is some child content that will overwrite the default slot content</p>
</Widget>

`<slot name="`name`">`

Named slots allow consumers to target specific areas. They can also have fallback content.

<!-- Widget.svelte -->
<div>
	<slot name="header">No header was provided</slot>
	<p>Some content between header and footer</p>
	<slot name="footer"></slot>
</div>

<!-- App.svelte -->
<Widget>
	<h1 slot="header">Hello</h1>
	<p slot="footer">Copyright (c) 2019 Svelte Industries</p>
</Widget>

Components can be placed in a named slot using the syntax <Component slot="name" />. In order to place content in a slot without using a wrapper element, you can use the special element <svelte:fragment>.

<!-- Widget.svelte -->
<div>
	<slot name="header">No header was provided</slot>
	<p>Some content between header and footer</p>
	<slot name="footer"></slot>
</div>

<!-- App.svelte -->
<Widget>
	<HeaderComponent slot="header" />
	<svelte:fragment slot="footer">
		<p>All rights reserved.</p>
		<p>Copyright (c) 2019 Svelte Industries</p>
	</svelte:fragment>
</Widget>

`$$slots`

$$slots is an object whose keys are the names of the slots passed into the component by the parent. If the parent does not pass in a slot with a particular name, that name will not be present in $$slots. This allows components to render a slot (and other elements, like wrappers for styling) only if the parent provides it.

Note that explicitly passing in an empty named slot will add that slot's name to $$slots. For example, if a parent passes <div slot="title" /> to a child component, $$slots.title will be truthy within the child.

<!-- Card.svelte -->
<div>
	<slot name="title"></slot>
	{#if $$slots.description}
		<!-- This <hr> and slot will render only if a slot named "description" is provided. -->
		<hr>
		<slot name="description"></slot>
	{/if}
</div>

<!-- App.svelte -->
<Card>
	<h1 slot="title">Blog Post Title</h1>
	<!-- No slot named "description" was provided so the optional slot will not be rendered. -->
</Card>

`<slot key={`value`}>`

Slots can be rendered zero or more times, and can pass values back to the parent using props. The parent exposes the values to the slot template using the let: directive.

The usual shorthand rules apply — let:item is equivalent to let:item={item}, and <slot {item}> is equivalent to <slot item={item}>.

<!-- FancyList.svelte -->
<ul>
	{#each items as item}
		<li class="fancy">
			<slot prop={item}></slot>
		</li>
	{/each}
</ul>

<!-- App.svelte -->
<FancyList {items} let:prop={thing}>
	<div>{thing.text}</div>
</FancyList>

Named slots can also expose values. The let: directive goes on the element with the slot attribute.

<!-- FancyList.svelte -->
<ul>
	{#each items as item}
		<li class="fancy">
			<slot name="item" {item}></slot>
		</li>
	{/each}
</ul>

<slot name="footer"></slot>

<!-- App.svelte -->
<FancyList {items}>
	<div slot="item" let:item>{item.text}</div>
	<p slot="footer">Copyright (c) 2019 Svelte Industries</p>
</FancyList>

`<svelte:self>`

The <svelte:self> element allows a component to include itself, recursively.

It cannot appear at the top level of your markup; it must be inside an if or each block or passed to a component's slot to prevent an infinite loop.

<script>
	export let count;
</script>

{#if count > 0}
	<p>counting down... {count}</p>
	<svelte:self count="{count - 1}"/>
{:else}
	<p>lift-off!</p>
{/if}

`<svelte:component>`

<svelte:component this={expression}/>

The <svelte:component> element renders a component dynamically, using the component constructor specified as the this property. When the property changes, the component is destroyed and recreated.

If this is falsy, no component is rendered.

<svelte:component this={currentSelection.component} foo={bar}/>

`<svelte:element>`

<svelte:element this={expression}/>

The <svelte:element> element lets you render an element of a dynamically specified type. This is useful for example when displaying rich text content from a CMS. Any properties and event listeners present will be applied to the element.

The only supported binding is bind:this, since the element type specific bindings that Svelte does at build time (e.g. bind:value for input elements) do not work with a dynamic tag type.

If this has a nullish value, the element and its children will not be rendered.

If this is the name of a void tag (e.g., br) and <svelte:element> has child elements, a runtime error will be thrown in development mode.

<script>
	let tag = 'div';
	export let handler;
</script>

<svelte:element this={tag} on:click={handler}>Foo</svelte:element>

`<svelte:window>`

<svelte:window on:event={handler}/>

<svelte:window bind:prop={value}/>

The <svelte:window> element allows you to add event listeners to the window object without worrying about removing them when the component is destroyed, or checking for the existence of window when server-side rendering.

Unlike <svelte:self>, this element may only appear at the top level of your component and must never be inside a block or element.

<script>
	function handleKeydown(event) {
		alert(`pressed the ${event.key} key`);
	}
</script>

<svelte:window on:keydown={handleKeydown}/>

You can also bind to the following properties:

innerWidth
innerHeight
outerWidth
outerHeight
scrollX
scrollY
online — an alias for window.navigator.onLine

All except scrollX and scrollY are readonly.

<svelte:window bind:scrollY={y}/>

Note that the page will not be scrolled to the initial value to avoid accessibility issues. Only subsequent changes to the bound variable of scrollX and scrollY will cause scrolling. However, if the scrolling behaviour is desired, call scrollTo() in onMount().

`<svelte:body>`

<svelte:body on:event={handler}/>

Similarly to <svelte:window>, this element allows you to add listeners to events on document.body, such as mouseenter and mouseleave, which don't fire on window. It also lets you use actions on the <body> element.

As with <svelte:window>, this element may only appear the top level of your component and must never be inside a block or element.

<svelte:body
	on:mouseenter={handleMouseenter}
	on:mouseleave={handleMouseleave}
	use:someAction
/>

`<svelte:head>`

<svelte:head>...</svelte:head>

This element makes it possible to insert elements into document.head. During server-side rendering, head content is exposed separately to the main html content.

As with <svelte:window> and <svelte:body>, this element may only appear at the top level of your component and must never be inside a block or element.

<svelte:head>
	<link rel="stylesheet" href="/tutorial/dark-theme.css">
</svelte:head>

`<svelte:options>`

<svelte:options option={value}/>

The <svelte:options> element provides a place to specify per-component compiler options, which are detailed in the compiler section. The possible options are:

immutable={true} — you never use mutable data, so the compiler can do simple referential equality checks to determine if values have changed
immutable={false} — the default. Svelte will be more conservative about whether or not mutable objects have changed
accessors={true} — adds getters and setters for the component's props
accessors={false} — the default
namespace="..." — the namespace where this component will be used, most commonly "svg"; use the "foreign" namespace to opt out of case-insensitive attribute names and HTML-specific warnings
tag="..." — the name to use when compiling this component as a custom element

<svelte:options tag="my-custom-element"/>

`<svelte:fragment>`

The <svelte:fragment> element allows you to place content in a named slot without wrapping it in a container DOM element. This keeps the flow layout of your document intact.

<!-- Widget.svelte -->
<div>
	<slot name="header">No header was provided</slot>
	<p>Some content between header and footer</p>
	<slot name="footer"></slot>
</div>

<!-- App.svelte -->
<Widget>
	<h1 slot="header">Hello</h1>
	<svelte:fragment slot="footer">
		<p>All rights reserved.</p>
		<p>Copyright (c) 2019 Svelte Industries</p>
	</svelte:fragment>
</Widget>

Run time

`svelte`

The svelte package exposes lifecycle functions and the context API.

`onMount`

onMount(callback: () => void)

onMount(callback: () => () => void)

The onMount function schedules a callback to run as soon as the component has been mounted to the DOM. It must be called during the component's initialisation (but doesn't need to live inside the component; it can be called from an external module).

onMount does not run inside a server-side component.

<script>
	import { onMount } from 'svelte';

	onMount(() => {
		console.log('the component has mounted');
	});
</script>

If a function is returned from onMount, it will be called when the component is unmounted.

<script>
	import { onMount } from 'svelte';

	onMount(() => {
		const interval = setInterval(() => {
			console.log('beep');
		}, 1000);

		return () => clearInterval(interval);
	});
</script>

This behaviour will only work when the function passed to onMount synchronously returns a value. async functions always return a Promise, and as such cannot synchronously return a function.

`beforeUpdate`

beforeUpdate(callback: () => void)

Schedules a callback to run immediately before the component is updated after any state change.

The first time the callback runs will be before the initial onMount

<script>
	import { beforeUpdate } from 'svelte';

	beforeUpdate(() => {
		console.log('the component is about to update');
	});
</script>

`afterUpdate`

afterUpdate(callback: () => void)

Schedules a callback to run immediately after the component has been updated.

The first time the callback runs will be after the initial onMount

<script>
	import { afterUpdate } from 'svelte';

	afterUpdate(() => {
		console.log('the component just updated');
	});
</script>

`onDestroy`

onDestroy(callback: () => void)

Schedules a callback to run immediately before the component is unmounted.

Out of onMount, beforeUpdate, afterUpdate and onDestroy, this is the only one that runs inside a server-side component.

<script>
	import { onDestroy } from 'svelte';

	onDestroy(() => {
		console.log('the component is being destroyed');
	});
</script>

`tick`

promise: Promise = tick()

Returns a promise that resolves once any pending state changes have been applied, or in the next microtask if there are none.

<script>
	import { beforeUpdate, tick } from 'svelte';

	beforeUpdate(async () => {
		console.log('the component is about to update');
		await tick();
		console.log('the component just updated');
	});
</script>

`setContext`

setContext(key: any, context: any)

Associates an arbitrary context object with the current component and the specified key and returns that object. The context is then available to children of the component (including slotted content) with getContext.

Like lifecycle functions, this must be called during component initialisation.

<script>
	import { setContext } from 'svelte';

	setContext('answer', 42);
</script>

Context is not inherently reactive. If you need reactive values in context then you can pass a store into context, which will be reactive.

`getContext`

context: any = getContext(key: any)

Retrieves the context that belongs to the closest parent component with the specified key. Must be called during component initialisation.

<script>
	import { getContext } from 'svelte';

	const answer = getContext('answer');
</script>

`hasContext`

hasContext: boolean = hasContext(key: any)

Checks whether a given key has been set in the context of a parent component. Must be called during component initialisation.

<script>
	import { hasContext } from 'svelte';

	if (hasContext('answer')) {
		// do something
	}
</script>

`getAllContexts`

contexts: Map<any, any> = getAllContexts()

Retrieves the whole context map that belongs to the closest parent component. Must be called during component initialisation. Useful, for example, if you programmatically create a component and want to pass the existing context to it.

<script>
	import { getAllContexts } from 'svelte';

	const contexts = getAllContexts();
</script>

`createEventDispatcher`

dispatch: ((name: string, detail?: any, options?: DispatchOptions) => boolean) = createEventDispatcher();

Creates an event dispatcher that can be used to dispatch component events. Event dispatchers are functions that can take two arguments: name and detail.

Component events created with createEventDispatcher create a CustomEvent. These events do not bubble. The detail argument corresponds to the CustomEvent.detail property and can contain any type of data.

<script>
	import { createEventDispatcher } from 'svelte';

	const dispatch = createEventDispatcher();
</script>

<button on:click="{() => dispatch('notify', 'detail value')}">Fire Event</button>

Events dispatched from child components can be listened to in their parent. Any data provided when the event was dispatched is available on the detail property of the event object.

<script>
	function callbackFunction(event) {
		console.log(`Notify fired! Detail: ${event.detail}`)
	}
</script>

<Child on:notify="{callbackFunction}"/>

Events can be cancelable by passing a third parameter to the dispatch function. The function returns false if the event is cancelled with event.preventDefault(), otherwise it returns true.

<script>
	import { createEventDispatcher } from 'svelte';

	const dispatch = createEventDispatcher();

	function notify() {
		const shouldContinue = dispatch('notify', 'detail value', { cancelable: true });
		if (shouldContinue) {
			// no one called preventDefault
		} else {
			// a listener called preventDefault
		}
	}
</script>

`svelte/store`

The svelte/store module exports functions for creating readable, writable and derived stores.

Keep in mind that you don't have to use these functions to enjoy the reactive $store syntax in your components. Any object that correctly implements .subscribe, unsubscribe, and (optionally) .set is a valid store, and will work both with the special syntax, and with Svelte's built-in derived stores.

This makes it possible to wrap almost any other reactive state handling library for use in Svelte. Read more about the store contract to see what a correct implementation looks like.

`writable`

store = writable(value?: any)

store = writable(value?: any, start?: (set: (value: any) => void) => () => void)

Function that creates a store which has values that can be set from 'outside' components. It gets created as an object with additional set and update methods.

set is a method that takes one argument which is the value to be set. The store value gets set to the value of the argument if the store value is not already equal to it.

update is a method that takes one argument which is a callback. The callback takes the existing store value as its argument and returns the new value to be set to the store.

import { writable } from 'svelte/store';

const count = writable(0);

count.subscribe(value => {
	console.log(value);
}); // logs '0'

count.set(1); // logs '1'

count.update(n => n + 1); // logs '2'

If a function is passed as the second argument, it will be called when the number of subscribers goes from zero to one (but not from one to two, etc). That function will be passed a set function which changes the value of the store. It must return a stop function that is called when the subscriber count goes from one to zero.

import { writable } from 'svelte/store';

const count = writable(0, () => {
	console.log('got a subscriber');
	return () => console.log('no more subscribers');
});

count.set(1); // does nothing

const unsubscribe = count.subscribe(value => {
	console.log(value);
}); // logs 'got a subscriber', then '1'

unsubscribe(); // logs 'no more subscribers'

Note that the value of a writable is lost when it is destroyed, for example when the page is refreshed. However, you can write your own logic to sync the value to for example the localStorage.

`readable`

store = readable(value?: any, start?: (set: (value: any) => void) => () => void)

Creates a store whose value cannot be set from 'outside', the first argument is the store's initial value, and the second argument to readable is the same as the second argument to writable.

import { readable } from 'svelte/store';

const time = readable(null, set => {
	set(new Date());

	const interval = setInterval(() => {
		set(new Date());
	}, 1000);

	return () => clearInterval(interval);
});

`derived`

store = derived(a, callback: (a: any) => any)

store = derived(a, callback: (a: any, set: (value: any) => void) => void | () => void, initial_value: any)

store = derived([a, ...b], callback: ([a: any, ...b: any[]]) => any)

store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void) => void | () => void, initial_value: any)

Derives a store from one or more other stores. The callback runs initially when the first subscriber subscribes and then whenever the store dependencies change.

In the simplest version, derived takes a single store, and the callback returns a derived value.

import { derived } from 'svelte/store';

const doubled = derived(a, $a => $a * 2);

The callback can set a value asynchronously by accepting a second argument, set, and calling it when appropriate.

In this case, you can also pass a third argument to derived — the initial value of the derived store before set is first called.

import { derived } from 'svelte/store';

const delayed = derived(a, ($a, set) => {
	setTimeout(() => set($a), 1000);
}, 'one moment...');

If you return a function from the callback, it will be called when a) the callback runs again, or b) the last subscriber unsubscribes.

import { derived } from 'svelte/store';

const tick = derived(frequency, ($frequency, set) => {
	const interval = setInterval(() => {
	  set(Date.now());
	}, 1000 / $frequency);

	return () => {
		clearInterval(interval);
	};
}, 'one moment...');

In both cases, an array of arguments can be passed as the first argument instead of a single store.

import { derived } from 'svelte/store';

const summed = derived([a, b], ([$a, $b]) => $a + $b);

const delayed = derived([a, b], ([$a, $b], set) => {
	setTimeout(() => set($a + $b), 1000);
});

`get`

value: any = get(store)

Generally, you should read the value of a store by subscribing to it and using the value as it changes over time. Occasionally, you may need to retrieve the value of a store to which you're not subscribed. get allows you to do so.

This works by creating a subscription, reading the value, then unsubscribing. It's therefore not recommended in hot code paths.

import { get } from 'svelte/store';

const value = get(store);

`svelte/motion`

The svelte/motion module exports two functions, tweened and spring, for creating writable stores whose values change over time after set and update, rather than immediately.

`tweened`

store = tweened(value: any, options)

Tweened stores update their values over a fixed duration. The following options are available:

delay (number, default 0) — milliseconds before starting
duration (number | function, default 400) — milliseconds the tween lasts
easing (function, default t => t) — an easing function
interpolate (function) — see below

store.set and store.update can accept a second options argument that will override the options passed in upon instantiation.

Both functions return a Promise that resolves when the tween completes. If the tween is interrupted, the promise will never resolve.

Out of the box, Svelte will interpolate between two numbers, two arrays or two objects (as long as the arrays and objects are the same 'shape', and their 'leaf' properties are also numbers).

<script>
	import { tweened } from 'svelte/motion';
	import { cubicOut } from 'svelte/easing';

	const size = tweened(1, {
		duration: 300,
		easing: cubicOut
	});

	function handleClick() {
		// this is equivalent to size.update(n => n + 1)
		$size += 1;
	}
</script>

<button
	on:click={handleClick}
	style="transform: scale({$size}); transform-origin: 0 0"
>embiggen</button>

If the initial value is undefined or null, the first value change will take effect immediately. This is useful when you have tweened values that are based on props, and don't want any motion when the component first renders.

const size = tweened(undefined, {
	duration: 300,
	easing: cubicOut
});

$: $size = big ? 100 : 10;

The interpolate option allows you to tween between any arbitrary values. It must be an (a, b) => t => value function, where a is the starting value, b is the target value, t is a number between 0 and 1, and value is the result. For example, we can use the d3-interpolate package to smoothly interpolate between two colours.

<script>
	import { interpolateLab } from 'd3-interpolate';
	import { tweened } from 'svelte/motion';

	const colors = [
		'rgb(255, 62, 0)',
		'rgb(64, 179, 255)',
		'rgb(103, 103, 120)'
	];

	const color = tweened(colors[0], {
		duration: 800,
		interpolate: interpolateLab
	});
</script>

{#each colors as c}
	<button
		style="background-color: {c}; color: white; border: none;"
		on:click="{e => color.set(c)}"
	>{c}</button>
{/each}

<h1 style="color: {$color}">{$color}</h1>

`spring`

store = spring(value: any, options)

A spring store gradually changes to its target value based on its stiffness and damping parameters. Whereas tweened stores change their values over a fixed duration, spring stores change over a duration that is determined by their existing velocity, allowing for more natural-seeming motion in many situations. The following options are available:

stiffness (number, default 0.15) — a value between 0 and 1 where higher means a 'tighter' spring
damping (number, default 0.8) — a value between 0 and 1 where lower means a 'springier' spring
precision (number, default 0.01) — determines the threshold at which the spring is considered to have 'settled', where lower means more precise

All of the options above can be changed while the spring is in motion, and will take immediate effect.

const size = spring(100);
size.stiffness = 0.3;
size.damping = 0.4;
size.precision = 0.005;

As with tweened stores, set and update return a Promise that resolves if the spring settles.

Both set and update can take a second argument — an object with hard or soft properties. { hard: true } sets the target value immediately; { soft: n } preserves existing momentum for n seconds before settling. { soft: true } is equivalent to { soft: 0.5 }.

const coords = spring({ x: 50, y: 50 });
// updates the value immediately
coords.set({ x: 100, y: 200 }, { hard: true });
// preserves existing momentum for 1s
coords.update(
	(target_coords, coords) => {
		return { x: target_coords.x, y: coords.y };
	},
	{ soft: 1 }
);

See a full example on the spring tutorial.

<script>
	import { spring } from 'svelte/motion';

	const coords = spring({ x: 50, y: 50 }, {
		stiffness: 0.1,
		damping: 0.25
	});
</script>

If the initial value is undefined or null, the first value change will take effect immediately, just as with tweened values (see above).

const size = spring();
$: $size = big ? 100 : 10;

`svelte/transition`

The svelte/transition module exports seven functions: fade, blur, fly, slide, scale, draw and crossfade. They are for use with Svelte transitions.

`fade`

transition:fade={params}

in:fade={params}

out:fade={params}

Animates the opacity of an element from 0 to the current opacity for in transitions and from the current opacity to 0 for out transitions.

fade accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number, default 400) — milliseconds the transition lasts
easing (function, default linear) — an easing function

You can see the fade transition in action in the transition tutorial.

<script>
	import { fade } from 'svelte/transition';
</script>

{#if condition}
	<div transition:fade="{{delay: 250, duration: 300}}">
		fades in and out
	</div>
{/if}

`blur`

transition:blur={params}

in:blur={params}

out:blur={params}

Animates a blur filter alongside an element's opacity.

blur accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number, default 400) — milliseconds the transition lasts
easing (function, default cubicInOut) — an easing function
opacity (number, default 0) - the opacity value to animate out to and in from
amount (number, default 5) - the size of the blur in pixels

<script>
	import { blur } from 'svelte/transition';
</script>

{#if condition}
	<div transition:blur="{{amount: 10}}">
		fades in and out
	</div>
{/if}

`fly`

transition:fly={params}

in:fly={params}

out:fly={params}

Animates the x and y positions and the opacity of an element. in transitions animate from an element's current (default) values to the provided values, passed as parameters. out transitions animate from the provided values to an element's default values.

fly accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number, default 400) — milliseconds the transition lasts
easing (function, default cubicOut) — an easing function
x (number, default 0) - the x offset to animate out to and in from
y (number, default 0) - the y offset to animate out to and in from
opacity (number, default 0) - the opacity value to animate out to and in from

You can see the fly transition in action in the transition tutorial.

<script>
	import { fly } from 'svelte/transition';
	import { quintOut } from 'svelte/easing';
</script>

{#if condition}
	<div transition:fly="{{delay: 250, duration: 300, x: 100, y: 500, opacity: 0.5, easing: quintOut}}">
		flies in and out
	</div>
{/if}

`slide`

transition:slide={params}

in:slide={params}

out:slide={params}

Slides an element in and out.

slide accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number, default 400) — milliseconds the transition lasts
easing (function, default cubicOut) — an easing function

<script>
	import { slide } from 'svelte/transition';
	import { quintOut } from 'svelte/easing';
</script>

{#if condition}
	<div transition:slide="{{delay: 250, duration: 300, easing: quintOut }}">
		slides in and out
	</div>
{/if}

`scale`

transition:scale={params}

in:scale={params}

out:scale={params}

Animates the opacity and scale of an element. in transitions animate from an element's current (default) values to the provided values, passed as parameters. out transitions animate from the provided values to an element's default values.

scale accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number, default 400) — milliseconds the transition lasts
easing (function, default cubicOut) — an easing function
start (number, default 0) - the scale value to animate out to and in from
opacity (number, default 0) - the opacity value to animate out to and in from

<script>
	import { scale } from 'svelte/transition';
	import { quintOut } from 'svelte/easing';
</script>

{#if condition}
	<div transition:scale="{{duration: 500, delay: 500, opacity: 0.5, start: 0.5, easing: quintOut}}">
		scales in and out
	</div>
{/if}

`draw`

transition:draw={params}

in:draw={params}

out:draw={params}

Animates the stroke of an SVG element, like a snake in a tube. in transitions begin with the path invisible and draw the path to the screen over time. out transitions start in a visible state and gradually erase the path. draw only works with elements that have a getTotalLength method, like <path> and <polyline>.

draw accepts the following parameters:

delay (number, default 0) — milliseconds before starting
speed (number, default undefined) - the speed of the animation, see below.
duration (number | function, default 800) — milliseconds the transition lasts
easing (function, default cubicInOut) — an easing function

The speed parameter is a means of setting the duration of the transition relative to the path's length. It is a modifier that is applied to the length of the path: duration = length / speed. A path that is 1000 pixels with a speed of 1 will have a duration of 1000ms, setting the speed to 0.5 will double that duration and setting it to 2 will halve it.

<script>
	import { draw } from 'svelte/transition';
	import { quintOut } from 'svelte/easing';
</script>

<svg viewBox="0 0 5 5" xmlns="http://www.w3.org/2000/svg">
	{#if condition}
		<path transition:draw="{{duration: 5000, delay: 500, easing: quintOut}}"
					d="M2 1 h1 v1 h1 v1 h-1 v1 h-1 v-1 h-1 v-1 h1 z"
					fill="none"
					stroke="cornflowerblue"
					stroke-width="0.1px"
					stroke-linejoin="round"
		/>
	{/if}
</svg>

`crossfade`

The crossfade function creates a pair of transitions called send and receive. When an element is 'sent', it looks for a corresponding element being 'received', and generates a transition that transforms the element to its counterpart's position and fades it out. When an element is 'received', the reverse happens. If there is no counterpart, the fallback transition is used.

crossfade accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number | function, default 800) — milliseconds the transition lasts
easing (function, default cubicOut) — an easing function
fallback (function) — A fallback transition to use for send when there is no matching element being received, and for receive when there is no element being sent.

<script>
	import { crossfade } from 'svelte/transition';
	import { quintOut } from 'svelte/easing';

	const [send, receive] = crossfade({
		duration:1500,
		easing: quintOut
	});
</script>

{#if condition}
	<h1 in:send={{key}} out:receive={{key}}>BIG ELEM</h1>
{:else}
	<small in:send={{key}} out:receive={{key}}>small elem</small>
{/if}

`svelte/animate`

The svelte/animate module exports one function for use with Svelte animations.

`flip`

animate:flip={params}

The flip function calculates the start and end position of an element and animates between them, translating the x and y values. flip stands for First, Last, Invert, Play.

flip accepts the following parameters:

delay (number, default 0) — milliseconds before starting
duration (number | function, default d => Math.sqrt(d) * 120) — see below
easing (function, default cubicOut) — an easing function

duration can be provided as either:

a number, in milliseconds.
a function, distance: number => duration: number, receiving the distance the element will travel in pixels and returning the duration in milliseconds. This allows you to assign a duration that is relative to the distance travelled by each element.

You can see a full example on the animations tutorial

<script>
	import { flip } from 'svelte/animate';
	import { quintOut } from 'svelte/easing';

	let list = [1, 2, 3];
</script>

{#each list as n (n)}
	<div animate:flip="{{delay: 250, duration: 250, easing: quintOut}}">
		{n}
	</div>
{/each}

`svelte/easing`

Easing functions specify the rate of change over time and are useful when working with Svelte's built-in transitions and animations as well as the tweened and spring utilities. svelte/easing contains 31 named exports, a linear ease and 3 variants of 10 different easing functions: in, out and inOut.

You can explore the various eases using the ease visualiser in the examples section.

ease	in	out	inOut
back	`backIn`	`backOut`	`backInOut`
bounce	`bounceIn`	`bounceOut`	`bounceInOut`
circ	`circIn`	`circOut`	`circInOut`
cubic	`cubicIn`	`cubicOut`	`cubicInOut`
elastic	`elasticIn`	`elasticOut`	`elasticInOut`
expo	`expoIn`	`expoOut`	`expoInOut`
quad	`quadIn`	`quadOut`	`quadInOut`
quart	`quartIn`	`quartOut`	`quartInOut`
quint	`quintIn`	`quintOut`	`quintInOut`
sine	`sineIn`	`sineOut`	`sineInOut`

`svelte/register`

To render Svelte components in Node.js without bundling, use require('svelte/register'). After that, you can use require to include any .svelte file.

require('svelte/register');

const App = require('./App.svelte').default;

...

const { html, css, head } = App.render({ answer: 42 });

The .default is necessary because we're converting from native JavaScript modules to the CommonJS modules recognised by Node. Note that if your component imports JavaScript modules, they will fail to load in Node and you will need to use a bundler instead.

To set compile options, or to use a custom file extension, call the register hook as a function:

require('svelte/register')({
  extensions: ['.customextension'], // defaults to ['.html', '.svelte']
	preserveComments: true
});

Client-side component API

Creating a component

const component = new Component(options)

A client-side component — that is, a component compiled with generate: 'dom' (or the generate option left unspecified) is a JavaScript class.

import App from './App.svelte';

const app = new App({
	target: document.body,
	props: {
		// assuming App.svelte contains something like
		// `export let answer`:
		answer: 42
	}
});

The following initialisation options can be provided:

option	default	description
`target`	none	An `HTMLElement` or `ShadowRoot` to render to. This option is required
`anchor`	`null`	A child of `target` to render the component immediately before
`props`	`{}`	An object of properties to supply to the component
`context`	`new Map()`	A `Map` of root-level context key-value pairs to supply to the component
`hydrate`	`false`	See below
`intro`	`false`	If `true`, will play transitions on initial render, rather than waiting for subsequent state changes

Existing children of target are left where they are.

The hydrate option instructs Svelte to upgrade existing DOM (usually from server-side rendering) rather than creating new elements. It will only work if the component was compiled with the hydratable: true option. Hydration of <head> elements only works properly if the server-side rendering code was also compiled with hydratable: true, which adds a marker to each element in the <head> so that the component knows which elements it's responsible for removing during hydration.

Whereas children of target are normally left alone, hydrate: true will cause any children to be removed. For that reason, the anchor option cannot be used alongside hydrate: true.

The existing DOM doesn't need to match the component — Svelte will 'repair' the DOM as it goes.

import App from './App.svelte';

const app = new App({
	target: document.querySelector('#server-rendered-html'),
	hydrate: true
});

`$set`

component.$set(props)

Programmatically sets props on an instance. component.$set({ x: 1 }) is equivalent to x = 1 inside the component's <script> block.

Calling this method schedules an update for the next microtask — the DOM is not updated synchronously.

component.$set({ answer: 42 });

`$on`

component.$on(event, callback)

Causes the callback function to be called whenever the component dispatches an event.

A function is returned that will remove the event listener when called.

const off = app.$on('selected', event => {
	console.log(event.detail.selection);
});

off();

`$destroy`

component.$destroy()

Removes a component from the DOM and triggers any onDestroy handlers.

Component props

component.prop

component.prop = value

If a component is compiled with accessors: true, each instance will have getters and setters corresponding to each of the component's props. Setting a value will cause a synchronous update, rather than the default async update caused by component.$set(...).

By default, accessors is false, unless you're compiling as a custom element.

console.log(app.count);
app.count += 1;

Custom element API

Svelte components can also be compiled to custom elements (aka web components) using the customElement: true compiler option. You should specify a tag name for the component using the <svelte:options> element.

<svelte:options tag="my-element" />

<script>
	export let name = 'world';
</script>

<h1>Hello {name}!</h1>
<slot></slot>

Alternatively, use tag={null} to indicate that the consumer of the custom element should name it.

import MyElement from './MyElement.svelte';

customElements.define('my-element', MyElement);

Once a custom element has been defined, it can be used as a regular DOM element:

document.body.innerHTML = `
	<my-element>
		<p>This is some slotted content</p>
	</my-element>
`;

By default, custom elements are compiled with accessors: true, which means that any props are exposed as properties of the DOM element (as well as being readable/writable as attributes, where possible).

To prevent this, add accessors={false} to <svelte:options>.

const el = document.querySelector('my-element');

// get the current value of the 'name' prop
console.log(el.name);

// set a new value, updating the shadow DOM
el.name = 'everybody';

Custom elements can be a useful way to package components for consumption in a non-Svelte app, as they will work with vanilla HTML and JavaScript as well as most frameworks. There are, however, some important differences to be aware of:

Styles are encapsulated, rather than merely scoped. This means that any non-component styles (such as you might have in a global.css file) will not apply to the custom element, including styles with the :global(...) modifier
Instead of being extracted out as a separate .css file, styles are inlined into the component as a JavaScript string
Custom elements are not generally suitable for server-side rendering, as the shadow DOM is invisible until JavaScript loads
In Svelte, slotted content renders lazily. In the DOM, it renders eagerly. In other words, it will always be created even if the component's <slot> element is inside an {#if ...} block. Similarly, including a <slot> in an {#each ...} block will not cause the slotted content to be rendered multiple times
The let: directive has no effect
Polyfills are required to support older browsers

Server-side component API

const result = Component.render(...)

Unlike client-side components, server-side components don't have a lifespan after you render them — their whole job is to create some HTML and CSS. For that reason, the API is somewhat different.

A server-side component exposes a render method that can be called with optional props. It returns an object with head, html, and css properties, where head contains the contents of any <svelte:head> elements encountered.

You can import a Svelte component directly into Node using svelte/register.

require('svelte/register');

const App = require('./App.svelte').default;

const { head, html, css } = App.render({
	answer: 42
});

The .render() method accepts the following parameters:

parameter	default	description
`props`	`{}`	An object of properties to supply to the component
`options`	`{}`	An object of options

The options object takes in the following options:

option	default	description
`context`	`new Map()`	A `Map` of root-level context key-value pairs to supply to the component

const { head, html, css } = App.render(
	// props
	{ answer: 42 },
	// options
	{
		context: new Map([['context-key', 'context-value']])
	}
);

Compile time

Typically, you won't interact with the Svelte compiler directly, but will instead integrate it into your build system using a bundler plugin. The bundler plugin that the Svelte team most recommends and invests in is vite-plugin-svelte. The SvelteKit framework provides a setup leveraging vite-plugin-svelte to build applications as well as a tool for packaging Svelte component libraries. Svelte Society maintains a list of other bundler plugins for additional tools like Rollup and Webpack.

Nonetheless, it's useful to understand how to use the compiler, since bundler plugins generally expose compiler options to you.

`svelte.compile`

result: {
	js,
	css,
	ast,
	warnings,
	vars,
	stats
} = svelte.compile(source: string, options?: {...})

This is where the magic happens. svelte.compile takes your component source code, and turns it into a JavaScript module that exports a class.

const svelte = require('svelte/compiler');

const result = svelte.compile(source, {
	// options
});

The following options can be passed to the compiler. None are required:

option	default	description
`filename`	`null`	`string` used for debugging hints and sourcemaps. Your bundler plugin will set it automatically.
`name`	`"Component"`	`string` that sets the name of the resulting JavaScript class (though the compiler will rename it if it would otherwise conflict with other variables in scope). It will normally be inferred from `filename`.
`format`	`"esm"`	If `"esm"`, creates a JavaScript module (with `import` and `export`). If `"cjs"`, creates a CommonJS module (with `require` and `module.exports`), which is useful in some server-side rendering situations or for testing.
`generate`	`"dom"`	If `"dom"`, Svelte emits a JavaScript class for mounting to the DOM. If `"ssr"`, Svelte emits an object with a `render` method suitable for server-side rendering. If `false`, no JavaScript or CSS is returned; just metadata.
`errorMode`	`"throw"`	If `"throw"`, Svelte throws when a compilation error occurred. If `"warn"`, Svelte will treat errors as warnings and add them to the warning report.
`varsReport`	`"strict"`	If `"strict"`, Svelte returns a variables report with only variables that are not globals nor internals. If `"full"`, Svelte returns a variables report with all detected variables. If `false`, no variables report is returned.
`dev`	`false`	If `true`, causes extra code to be added to components that will perform runtime checks and provide debugging information during development.
`immutable`	`false`	If `true`, tells the compiler that you promise not to mutate any objects. This allows it to be less conservative about checking whether values have changed.
`hydratable`	`false`	If `true` when generating DOM code, enables the `hydrate: true` runtime option, which allows a component to upgrade existing DOM rather than creating new DOM from scratch. When generating SSR code, this adds markers to `<head>` elements so that hydration knows which to replace.
`legacy`	`false`	If `true`, generates code that will work in IE9 and IE10, which don't support things like `element.dataset`.
`accessors`	`false`	If `true`, getters and setters will be created for the component's props. If `false`, they will only be created for readonly exported values (i.e. those declared with `const`, `class` and `function`). If compiling with `customElement: true` this option defaults to `true`.
`customElement`	`false`	If `true`, tells the compiler to generate a custom element constructor instead of a regular Svelte component.
`tag`	`null`	A `string` that tells Svelte what tag name to register the custom element with. It must be a lowercase alphanumeric string with at least one hyphen, e.g. `"my-element"`.
`css`	`'injected'`	If `'injected'` (formerly `true`), styles will be included in the JavaScript class and injected at runtime for the components actually rendered. If `'external'` (formerly `false`), the CSS will be returned in the `css` field of the compilation result. Most Svelte bundler plugins will set this to `'external'` and use the CSS that is statically generated for better performance, as it will result in smaller JavaScript bundles and the output can be served as cacheable `.css` files. If `'none'`, styles are completely avoided and no CSS output is generated.
`cssHash`	See right	A function that takes a `{ hash, css, name, filename }` argument and returns the string that is used as a classname for scoped CSS. It defaults to returning `svelte-${hash(css)}`
`loopGuardTimeout`	0	A `number` that tells Svelte to break the loop if it blocks the thread for more than `loopGuardTimeout` ms. This is useful to prevent infinite loops. Only available when `dev: true`
`preserveComments`	`false`	If `true`, your HTML comments will be preserved during server-side rendering. By default, they are stripped out.
`preserveWhitespace`	`false`	If `true`, whitespace inside and between elements is kept as you typed it, rather than removed or collapsed to a single space where possible.
`sourcemap`	`object \| string`	An initial sourcemap that will be merged into the final output sourcemap. This is usually the preprocessor sourcemap.
`enableSourcemap`	`boolean \| { js: boolean; css: boolean; }`	If `true`, Svelte generate sourcemaps for components. Use an object with `js` or `css` for more granular control of sourcemap generation. By default, this is `true`.
`outputFilename`	`null`	A `string` used for your JavaScript sourcemap.
`cssOutputFilename`	`null`	A `string` used for your CSS sourcemap.
`sveltePath`	`"svelte"`	The location of the `svelte` package. Any imports from `svelte` or `svelte/[module]` will be modified accordingly.
`namespace`	`"html"`	The namespace of the element; e.g., `"mathml"`, `"svg"`, `"foreign"`.

The returned result object contains the code for your component, along with useful bits of metadata.

const {
	js,
	css,
	ast,
	warnings,
	vars,
	stats
} = svelte.compile(source);

js and css are objects with the following properties:
- code is a JavaScript string
- map is a sourcemap with additional toString() and toUrl() convenience methods
ast is an abstract syntax tree representing the structure of your component.
warnings is an array of warning objects that were generated during compilation. Each warning has several properties:
- code is a string identifying the category of warning
- message describes the issue in human-readable terms
- start and end, if the warning relates to a specific location, are objects with line, column and character properties
- frame, if applicable, is a string highlighting the offending code with line numbers
vars is an array of the component's declarations, used by eslint-plugin-svelte3 for example. Each variable has several properties:
- name is self-explanatory
- export_name is the name the value is exported as, if it is exported (will match name unless you do export...as)
- injected is true if the declaration is injected by Svelte, rather than in the code you wrote
- module is true if the value is declared in a context="module" script
- mutated is true if the value's properties are assigned to inside the component
- reassigned is true if the value is reassigned inside the component
- referenced is true if the value is used in the template
- referenced_from_script is true if the value is used in the <script> outside the declaration
- writable is true if the value was declared with let or var (but not const, class or function)
stats is an object used by the Svelte developer team for diagnosing the compiler. Avoid relying on it to stay the same!

`svelte.parse`

ast: object = svelte.parse(
	source: string,
	options?: {
		filename?: string,
		customElement?: boolean
	}
)

The parse function parses a component, returning only its abstract syntax tree. Unlike compiling with the generate: false option, this will not perform any validation or other analysis of the component beyond parsing it. Note that the returned AST is not considered public API, so breaking changes could occur at any point in time.

const svelte = require('svelte/compiler');

const ast = svelte.parse(source, { filename: 'App.svelte' });

`svelte.preprocess`

A number of community-maintained preprocessing plugins are available to allow you to use Svelte with tools like TypeScript, PostCSS, SCSS, and Less.

You can write your own preprocessor using the svelte.preprocess API.

result: {
	code: string,
	dependencies: Array<string>
} = await svelte.preprocess(
	source: string,
	preprocessors: Array<{
		markup?: (input: { content: string, filename: string }) => Promise<{
			code: string,
			dependencies?: Array<string>
		}>,
		script?: (input: { content: string, markup: string, attributes: Record<string, string>, filename: string }) => Promise<{
			code: string,
			dependencies?: Array<string>
		}>,
		style?: (input: { content: string, markup: string, attributes: Record<string, string>, filename: string }) => Promise<{
			code: string,
			dependencies?: Array<string>
		}>
	}>,
	options?: {
		filename?: string
	}
)

The preprocess function provides convenient hooks for arbitrarily transforming component source code. For example, it can be used to convert a <style lang="sass"> block into vanilla CSS.

The first argument is the component source code. The second is an array of preprocessors (or a single preprocessor, if you only have one), where a preprocessor is an object with markup, script and style functions, each of which is optional.

Each markup, script or style function must return an object (or a Promise that resolves to an object) with a code property, representing the transformed source code, and an optional array of dependencies.

The markup function receives the entire component source text, along with the component's filename if it was specified in the third argument.

Preprocessor functions should additionally return a map object alongside code and dependencies, where map is a sourcemap representing the transformation.

const svelte = require('svelte/compiler');
const MagicString = require('magic-string');

const { code } = await svelte.preprocess(source, {
	markup: ({ content, filename }) => {
		const pos = content.indexOf('foo');
		if(pos < 0) {
			return { code: content }
		}
		const s = new MagicString(content, { filename })
		s.overwrite(pos, pos + 3, 'bar', { storeName: true })
		return {
			code: s.toString(),
			map: s.generateMap()
		}
	}
}, {
	filename: 'App.svelte'
});

The script and style functions receive the contents of <script> and <style> elements respectively (content) as well as the entire component source text (markup). In addition to filename, they get an object of the element's attributes.

If a dependencies array is returned, it will be included in the result object. This is used by packages like rollup-plugin-svelte to watch additional files for changes, in the case where your <style> tag has an @import (for example).

const svelte = require('svelte/compiler');
const sass = require('node-sass');
const { dirname } = require('path');

const { code, dependencies } = await svelte.preprocess(source, {
	style: async ({ content, attributes, filename }) => {
		// only process <style lang="sass">
		if (attributes.lang !== 'sass') return;

		const { css, stats } = await new Promise((resolve, reject) => sass.render({
			file: filename,
			data: content,
			includePaths: [
				dirname(filename),
			],
		}, (err, result) => {
			if (err) reject(err);
			else resolve(result);
		}));

		return {
			code: css.toString(),
			dependencies: stats.includedFiles
		};
	}
}, {
	filename: 'App.svelte'
});

Multiple preprocessors can be used together. The output of the first becomes the input to the second. markup functions run first, then script and style.

const svelte = require('svelte/compiler');

const { code } = await svelte.preprocess(source, [
	{
		markup: () => {
			console.log('this runs first');
		},
		script: () => {
			console.log('this runs third');
		},
		style: () => {
			console.log('this runs fifth');
		}
	},
	{
		markup: () => {
			console.log('this runs second');
		},
		script: () => {
			console.log('this runs fourth');
		},
		style: () => {
			console.log('this runs sixth');
		}
	}
], {
	filename: 'App.svelte'
});

`svelte.walk`

walk(ast: Node, {
	enter(node: Node, parent: Node, prop: string, index: number)?: void,
	leave(node: Node, parent: Node, prop: string, index: number)?: void
})

The walk function provides a way to walk the abstract syntax trees generated by the parser, using the compiler's own built-in instance of estree-walker.

The walker takes an abstract syntax tree to walk and an object with two optional methods: enter and leave. For each node, enter is called (if present). Then, unless this.skip() is called during enter, each of the children are traversed, and then leave is called on the node.

const svelte = require('svelte/compiler');
svelte.walk(ast, {
	enter(node, parent, prop, index) {
		do_something(node);
		if (should_skip_children(node)) {
			this.skip();
		}
	},
	leave(node, parent, prop, index) {
		do_something_else(node);
	}
});

`svelte.VERSION`

The current version, as set in package.json.

const svelte = require('svelte/compiler');
console.log(`running svelte version ${svelte.VERSION}`);

Accessibility warnings

Accessibility (shortened to a11y) isn't always easy to get right, but Svelte will help by warning you at compile time if you write inaccessible markup. However, keep in mind that many accessibility issues can only be identified at runtime using other automated tools and by manually testing your application.

Here is a list of accessibility checks Svelte will do for you.

`a11y-accesskey`

Enforce no accesskey on element. Access keys are HTML attributes that allow web developers to assign keyboard shortcuts to elements. Inconsistencies between keyboard shortcuts and keyboard commands used by screen reader and keyboard-only users create accessibility complications. To avoid complications, access keys should not be used.

<!-- A11y: Avoid using accesskey -->
<div accessKey='z'></div>

`a11y-aria-attributes`

Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example meta, html, script, style. This rule enforces that these DOM elements do not contain the aria-* props.

<!-- A11y: <meta> should not have aria-* attributes -->
<meta aria-hidden="false">

`a11y-autofocus`

Enforce that autofocus is not used on elements. Autofocusing elements can cause usability issues for sighted and non-sighted users alike.

<!-- A11y: Avoid using autofocus -->
<input autofocus>

`a11y-click-events-have-key-events`

Enforce on:click is accompanied by at least one of the following: onKeyUp, onKeyDown, onKeyPress. Coding for the keyboard is important for users with physical disabilities who cannot use a mouse, AT compatibility, and screenreader users.

This does not apply for interactive or hidden elements.

<!-- A11y: visible, non-interactive elements with an on:click event must be accompanied by an on:keydown, on:keyup, or on:keypress event. -->
<div on:click={() => {}} />

`a11y-distracting-elements`

Enforces that no distracting elements are used. Elements that can be visually distracting can cause accessibility issues with visually impaired users. Such elements are most likely deprecated, and should be avoided.

The following elements are visually distracting: <marquee> and <blink>.

<!-- A11y: Avoid <marquee> elements -->
<marquee />

`a11y-hidden`

Certain DOM elements are useful for screen reader navigation and should not be hidden.

<!-- A11y: <h2> element should not be hidden -->
<h2 aria-hidden="true">invisible header</h2>

`a11y-img-redundant-alt`

Enforce img alt attribute does not contain the word image, picture, or photo. Screen readers already announce img elements as an image. There is no need to use words such as image, photo, and/or picture.

<img src="foo" alt="Foo eating a sandwich." />

<!-- aria-hidden, won't be announced by screen reader -->
<img src="bar" aria-hidden="true" alt="Picture of me taking a photo of an image" />

<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="foo" alt="Photo of foo being weird." />

<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="bar" alt="Image of me at a bar!" />

<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="foo" alt="Picture of baz fixing a bug." />

`a11y-incorrect-aria-attribute-type`

Enforce that only the correct type of value is used for aria attributes. For example, aria-hidden should only receive a boolean.

<!-- A11y: The value of 'aria-hidden' must be exactly one of true or false -->
<div aria-hidden="yes"/>

`a11y-invalid-attribute`

Enforce that attributes important for accessibility have a valid value. For example, href should not be empty, '#', or javascript:.

<!-- A11y: '' is not a valid href attribute -->
<a href=''>invalid</a>

`a11y-label-has-associated-control`

Enforce that a label tag has a text label and an associated control.

There are two supported ways to associate a label with a control:

Wrapping a control in a label tag.
Adding for to a label and assigning it the ID of an input on the page.

<label for="id">B</label>

<label>C <input type="text" /></label>

<!-- A11y: A form label must be associated with a control. -->
<label>A</label>

`a11y-media-has-caption`

Providing captions for media is essential for deaf users to follow along. Captions should be a transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information. Not only is this important for accessibility, but can also be useful for all users in the case that the media is unavailable (similar to alt text on an image when an image is unable to load).

The captions should contain all important and relevant information to understand the corresponding media. This may mean that the captions are not a 1:1 mapping of the dialogue in the media content. However, captions are not necessary for video components with the muted attribute.

<video><track kind="captions"/></video>

<audio muted></audio>

<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
<video></video>

<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
<video><track /></video>

`a11y-misplaced-role`

<!-- A11y: <meta> should not have role attribute -->
<meta role="tooltip">

`a11y-misplaced-scope`

The scope attribute should only be used on <th> elements.

<!-- A11y: The scope attribute should only be used with <th> elements -->
<div scope="row" />

`a11y-missing-attribute`

Enforce that attributes required for accessibility are present on an element. This includes the following checks:

<a> should have an href (unless it's a fragment-defining tag)
<area> should have alt, aria-label, or aria-labelledby
<html> should have lang
<iframe> should have title
<img> should have alt
<object> should have title, aria-label, or aria-labelledby
<input type="image"> should have alt, aria-label, or aria-labelledby

<!-- A11y: <input type=\"image\"> element should have an alt, aria-label or aria-labelledby attribute -->
<input type="image">

<!-- A11y: <html> element should have a lang attribute -->
<html></html>

<!-- A11y: <a> element should have an href attribute -->
<a>text</a>

`a11y-missing-content`

Enforce that heading elements (h1, h2, etc.) and anchors have content and that the content is accessible to screen readers

<!-- A11y: <a> element should have child content -->
<a href='/foo'></a>

<!-- A11y: <h1> element should have child content -->
<h1></h1>

`a11y-mouse-events-have-key-events`

Enforce that on:mouseover and on:mouseout are accompanied by on:focus and on:blur, respectively. This helps to ensure that any functionality triggered by these mouse events is also accessible to keyboard users.

<!-- A11y: on:mouseover must be accompanied by on:focus -->
<div on:mouseover={handleMouseover} />

<!-- A11y: on:mouseout must be accompanied by on:blur -->
<div on:mouseout={handleMouseout} />

`a11y-no-redundant-roles`

Some HTML elements have default ARIA roles. Giving these elements an ARIA role that is already set by the browser has no effect and is redundant.

<!-- A11y: Redundant role 'button' -->
<button role="button" />

<!-- A11y: Redundant role 'img' -->
<img role="img" src="foo.jpg" />

`a11y-no-interactive-element-to-noninteractive-role`

WAI-ARIA roles should not be used to convert an interactive element to a non-interactive element. Non-interactive ARIA roles include article, banner, complementary, img, listitem, main, region and tooltip.

<!-- A11y: <textarea> cannot have role 'listitem' -->
<textarea role="listitem" />

`a11y-no-noninteractive-tabindex`

Tab key navigation should be limited to elements on the page that can be interacted with.

<!-- A11y: noninteractive element cannot have nonnegative tabIndex value -->
<div tabindex='0' />

`a11y-positive-tabindex`

Avoid positive tabindex property values. This will move elements out of the expected tab order, creating a confusing experience for keyboard users.

<!-- A11y: avoid tabindex values above zero -->
<div tabindex='1'/>

`a11y-role-has-required-aria-props`

Elements with ARIA roles must have all required attributes for that role.

<!-- A11y: A11y: Elements with the ARIA role "checkbox" must have the following attributes defined: "aria-checked" -->
<span role="checkbox" aria-labelledby="foo" tabindex="0"></span>

`a11y-structure`

Enforce that certain DOM elements have the correct structure.

<!-- A11y: <figcaption> must be an immediate child of <figure> -->
<div>
	<figcaption>Image caption</figcaption>
</div>

`a11y-unknown-aria-attribute`

Enforce that only known ARIA attributes are used. This is based on the WAI-ARIA States and Properties spec.

<!-- A11y: Unknown aria attribute 'aria-labeledby' (did you mean 'labelledby'?) -->
<input type="image" aria-labeledby="foo">

`a11y-unknown-role`

Elements with ARIA roles must use a valid, non-abstract ARIA role. A reference to role definitions can be found at WAI-ARIA site.

<!-- A11y: Unknown role 'toooltip' (did you mean 'tooltip'?) -->
<div role="toooltip"></div>

Svelte

Before we begin

Getting started

Component format

<script>

1. export creates a component prop

2. Assignments are 'reactive'

3. $: marks a statement as reactive

4. Prefix stores with $ to access their values

Store contract

<script context="module">

<style>

Template syntax

Tags

Attributes and props

Text expressions

Comments

{#if ...}

{#each ...}

{#await ...}

{#key ...}

{@html ...}

{@debug ...}

{@const ...}

Element directives

on:eventname

bind:property

Binding <select> value

Media element bindings

Block-level element bindings

bind:group

bind:this

class:name

style:property

use:action

transition:fn

Transition parameters

Custom transition functions

Transition events

in:fn/out:fn

animate:fn

Animation Parameters

Custom animation functions

Component directives

on:eventname

--style-props

bind:property

bind:this

<slot>

<slot name="name">

$$slots

<slot key={value}>

<svelte:self>

<svelte:component>

<svelte:element>

<svelte:window>

<svelte:body>

<svelte:head>

<svelte:options>

<svelte:fragment>

Run time

svelte

onMount

beforeUpdate

afterUpdate

onDestroy

tick

setContext

getContext

hasContext

getAllContexts

createEventDispatcher

svelte/store

writable

readable

derived

get

svelte/motion

tweened

spring

1. `export` creates a component prop

3. `$:` marks a statement as reactive

4. Prefix stores with `$` to access their values

Binding `<select>` value

`<slot>`

`<slot name="`name`">`

`$$slots`

`<slot key={`value`}>`

`<svelte:self>`

`<svelte:component>`

`<svelte:element>`

`<svelte:window>`

`<svelte:body>`

`<svelte:head>`

`<svelte:options>`

`<svelte:fragment>`

`svelte`

`onMount`

`beforeUpdate`

`afterUpdate`

`onDestroy`

`tick`

`setContext`

`getContext`

`hasContext`

`getAllContexts`

`createEventDispatcher`

`svelte/store`

`writable`

`readable`

`derived`

`get`

`svelte/motion`

`tweened`

`spring`

`svelte/transition`

`fade`

`blur`

`fly`

`slide`

`scale`

`draw`

`crossfade`

`svelte/animate`

`flip`

`svelte/easing`

`svelte/register`

`$set`

`$on`

`$destroy`

`svelte.compile`

`svelte.parse`

`svelte.preprocess`

`svelte.walk`

`svelte.VERSION`

`a11y-accesskey`

`a11y-aria-attributes`

`a11y-autofocus`

`a11y-click-events-have-key-events`

`a11y-distracting-elements`

`a11y-hidden`

`a11y-img-redundant-alt`

`a11y-incorrect-aria-attribute-type`

`a11y-invalid-attribute`

`a11y-label-has-associated-control`

`a11y-media-has-caption`

`a11y-misplaced-role`

`a11y-misplaced-scope`

`a11y-missing-attribute`

`a11y-missing-content`

`a11y-mouse-events-have-key-events`

`a11y-no-redundant-roles`

`a11y-no-interactive-element-to-noninteractive-role`

`a11y-no-noninteractive-tabindex`

`a11y-positive-tabindex`

`a11y-role-has-required-aria-props`

`a11y-structure`

`a11y-unknown-aria-attribute`

`a11y-unknown-role`