svelte/documentation/docs/02-template-syntax/08-bindings.md

---
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

```svelte
<!--- 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`.

```svelte
<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.

```svelte
<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`.

```svelte
<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](https://developer.mozilla.org/en-US/docs/Web/API/FileList). It is readonly.

```svelte
<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.

```svelte
<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).

```svelte
<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>`.

```svelte
<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.

```svelte
<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:

- [`innerHTML`](https://developer.mozilla.org/en-US/docs/Web/API/Element/innerHTML)
- [`innerText`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/innerText)
- [`textContent`](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent)

There are slight differences between each of these, read more about them [here](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent#Differences_from_innerText).

<!-- for some reason puts the comment and html on same line -->
<!-- prettier-ignore -->
```svelte
<div contenteditable="true" bind:innerHTML={html} />
```

`<details>` elements support binding to the `open` property.

```svelte
<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.

```svelte
<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

```svelte
<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](http://www.backalleycoder.com/2013/03/18/cross-browser-event-based-element-resize-detection/):

- `clientWidth`
- `clientHeight`
- `offsetWidth`
- `offsetHeight`

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

## bind:group

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

Inputs that work together can use `bind:group`.

```svelte
<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

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

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

```svelte
<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.

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

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

```svelte
<!--- 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

```svelte
bind:property={variable}
```

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

```svelte
<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:

```svelte
<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:

```svelte
<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.