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187 lines
6.1 KiB
187 lines
6.1 KiB
---
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title: Special elements
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---
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- [basically what we have in the docs today](https://svelte.dev/docs/special-elements)
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Some of Svelte's concepts need special elements. Those are prefixed with `svelte:` and listed here.
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## `<svelte:self>`
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The `<svelte:self>` element allows a component to include itself, recursively.
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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.
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```svelte
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<script>
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/** @type {number} */
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export let count;
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</script>
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{#if count > 0}
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<p>counting down... {count}</p>
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<svelte:self count={count - 1} />
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{:else}
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<p>lift-off!</p>
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{/if}
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```
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## `<svelte:component>`
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```svelte
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<svelte:component this={expression} />
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```
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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.
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If `this` is falsy, no component is rendered.
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```svelte
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<svelte:component this={currentSelection.component} foo={bar} />
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```
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## `<svelte:element>`
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```svelte
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<svelte:element this={expression} />
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```
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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.
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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.
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If `this` has a nullish value, the element and its children will not be rendered.
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If `this` is the name of a [void element](https://developer.mozilla.org/en-US/docs/Glossary/Void_element) (e.g., `br`) and `<svelte:element>` has child elements, a runtime error will be thrown in development mode.
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```svelte
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<script>
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let tag = 'div';
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export let handler;
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</script>
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<svelte:element this={tag} on:click={handler}>Foo</svelte:element>
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```
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## `<svelte:window>`
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```svelte
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<svelte:window on:event={handler} />
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```
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```svelte
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<svelte:window bind:prop={value} />
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```
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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.
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Unlike `<svelte:self>`, this element may only appear at the top level of your component and must never be inside a block or element.
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```svelte
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<script>
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/** @param {KeyboardEvent} event */
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function handleKeydown(event) {
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alert(`pressed the ${event.key} key`);
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}
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</script>
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<svelte:window on:keydown={handleKeydown} />
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```
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You can also bind to the following properties:
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- `innerWidth`
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- `innerHeight`
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- `outerWidth`
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- `outerHeight`
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- `scrollX`
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- `scrollY`
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- `online` — an alias for `window.navigator.onLine`
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- `devicePixelRatio`
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All except `scrollX` and `scrollY` are readonly.
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```svelte
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<svelte:window bind:scrollY={y} />
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```
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> 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()`.
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## `<svelte:document>`
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```svelte
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<svelte:document on:event={handler} />
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```
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```svelte
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<svelte:document bind:prop={value} />
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```
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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](/docs/element-directives#use-action) on `document`.
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As with `<svelte:window>`, this element may only appear the top level of your component and must never be inside a block or element.
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```svelte
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<svelte:document on:visibilitychange={handleVisibilityChange} use:someAction />
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```
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You can also bind to the following properties:
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- `activeElement`
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- `fullscreenElement`
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- `pointerLockElement`
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- `visibilityState`
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All are readonly.
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## `<svelte:body>`
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```svelte
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<svelte:body on:event={handler} />
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```
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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](/docs/element-directives#use-action) on the `<body>` element.
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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.
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```svelte
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<svelte:body on:mouseenter={handleMouseenter} on:mouseleave={handleMouseleave} use:someAction />
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```
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## `<svelte:head>`
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```svelte
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<svelte:head>...</svelte:head>
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```
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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.
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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.
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```svelte
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<svelte:head>
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<title>Hello world!</title>
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<meta name="description" content="This is where the description goes for SEO" />
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</svelte:head>
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```
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## `<svelte:options>`
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```svelte
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<svelte:options option={value} />
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```
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The `<svelte:options>` element provides a place to specify per-component compiler options, which are detailed in the [compiler section](/docs/svelte-compiler#compile). The possible options are:
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- `immutable={true}` — you never use mutable data, so the compiler can do simple referential equality checks to determine if values have changed
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- `immutable={false}` — the default. Svelte will be more conservative about whether or not mutable objects have changed
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- `accessors={true}` — adds getters and setters for the component's props
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- `accessors={false}` — the default
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- `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
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- `customElement="..."` — the name to use when compiling this component as a custom element
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```svelte
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<svelte:options customElement="my-custom-element" />
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```
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