svelte/documentation/docs/02-template-syntax/07-special-elements.md

title
Special elements

<slot>

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

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

<slot prop={value} />

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 />
<!-- this component will render the default content -->

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

Note: If you want to render regular <slot> element, You can use <svelte:element this="slot" />.

<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" />
</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" />
</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" />
	{#if $$slots.description}
		<!-- This <hr> and slot will render only if a slot named "description" is provided. -->
		<hr />
		<slot name="description" />
	{/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} />
		</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} />
		</li>
	{/each}
</ul>

<slot name="footer" />

<!-- 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>
	/** @type {number} */
	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 element (e.g., br) and <svelte:element> has child elements, a runtime error will be thrown in development mode.

<script>
	let tag = 'div';

	/** @type {(e: MouseEvent) => void} */
	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>
	/** @param {KeyboardEvent} event */
	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
• devicePixelRatio

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:document>

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

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

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

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:document on:visibilitychange={handleVisibilityChange} use:someAction />

You can also bind to the following properties:

• fullscreenElement
• visibilityState

All are readonly.

<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> and <svelte:document>, 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>, <svelte:document> 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>
	<title>Hello world!</title>
	<meta name="description" content="This is where the description goes for SEO" />
</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
• customElement="..." — the name to use when compiling this component as a custom element

<svelte:options customElement="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" />
</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>