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Template syntax

Tags

A lowercase tag, like <div>, denotes a regular HTML element. A capitalised tag, such as <Widget> or <Namespace.Widget>, indicates a component.

<script>
  import Widget from './Widget.svelte';
</script>

<div>
  <Widget/>
</div>

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}

(Blocks don't have to wrap elements, they can also wrap text within elements!)

{#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
  • stopImmediatePropagation - calls event.stopImmediatePropagation(), preventing other listeners of the same event from being fired.
  • 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. The bound variable is an array with an entry corresponding to the value property of each selected <option>.

<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 the following bindings:

There are slight differences between each of these, read more about them here.

<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 — seven 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
  • readyState (readonly) — number between (and including) 0 and 4

...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:readyState
  bind:currentTime
  bind:playbackRate
  bind:paused
  bind:volume
  bind:muted
  bind:videoWidth
  bind:videoHeight
></video>

Image element bindings

Image elements (<img>) have two readonly bindings:

  • naturalWidth (readonly) — the original width of the image, available after the image has loaded
  • naturalHeight (readonly) — the original height of the image, available after the image has loaded
<img
  bind:naturalWidth
  bind:naturalHeight
></img>

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>

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"></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 element (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:document>

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

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
/>

<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>
  <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>