svelte

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title
Bindings

how for dom elements
list of all bindings
how for components

Most of the time a clear separation between data flowing down and events going up is worthwhile and results in more robust apps. But in some cases - especially when interacting with form elements - it's more ergonomic to declare a two way binding. Svelte provides many element bindings out of the box, and also allows component bindings.

bind:property for elements

<!--- copy: false --->
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} />

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

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

<input bind:value />
<!-- equivalent to
<input bind:value={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. When you want to update the files programmatically, you always need to use a FileList object. Currently FileList objects cannot be constructed directly, so you need to create a new DataTransfer object and get files from there.

<script>
	let files = $state();

	function clear() {
		files = new DataTransfer().files; // null or undefined does not work
	}
</script>

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

FileList objects also cannot be modified, so if you want to e.g. delete a single file from the list, you need to create a new DataTransfer object and add the files you want to keep.

DataTransfer may not be available in server-side JS runtimes. Leaving the state that is bound to files uninitialized prevents potential errors if components are server-side rendered.

If you're using bind: directives together with on event attributes, the binding will always fire before the event attribute.

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

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

<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
/>

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

<!--- copy: false --->
bind:group={variable}

Inputs that work together can use bind:group.

<script>
	let tortilla = 'Plain';

	/** @type {Array<string>} */
	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:group only works if the inputs are in the same Svelte component.

bind:this

<!--- copy: false --->
bind:this={dom_node}

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

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

	/** @type {HTMLCanvasElement} */
	let canvasElement;

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

<canvas bind:this={canvasElement} />

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

<!--- App.svelte --->
<ShoppingCart bind:this={cart} />

<button onclick={() => cart.empty()}> Empty shopping cart </button>

<!--- ShoppingCart.svelte --->
<script>
	// All instance exports are available on the instance object
	export function empty() {
		// ...
	}
</script>

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

bind:property for components

bind:property={variable}

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

<Keypad bind:value={pin} />

While Svelte props are reactive without binding, that reactivity only flows downward into the component by default. Using bind:property allows changes to the property from within the component to flow back up out of the component.

To mark a property as bindable, use the $bindable rune:

<script>
	let { readonlyProperty, bindableProperty = $bindable() } = $props();
</script>

Declaring a property as bindable means it can be used using bind:, not that it must be used using bind:.

Bindable properties can have a fallback value:

<script>
	let { bindableProperty = $bindable('fallback value') } = $props();
</script>

This fallback value only applies when the property is not bound. When the property is bound and a fallback value is present, the parent is expected to provide a value other than undefined, else a runtime error is thrown. This prevents hard-to-reason-about situations where it's unclear which value should apply.

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