---
title: bind:
---

Data ordinarily flows down, from parent to child. The `bind:` directive allows data to flow the other way, from child to parent.

The general syntax is `bind:property={expression}`, where `expression` is an _lvalue_ (i.e. a variable or an object property). When the expression is an identifier with the same name as the property, we can omit the expression — in other words these are equivalent:

<!-- prettier-ignore -->
```svelte
<input bind:value={value} />
<input bind:value />
```

Svelte creates an event listener that updates the bound value. If an element already has a listener for the same event, that listener will be fired before the bound value is updated.

Most bindings are _two-way_, meaning that changes to the value will affect the element and vice versa. A few bindings are _readonly_, meaning that changing their value will have no effect on the element.

## `<input bind:value>`

A `bind:value` directive on an `<input>` element binds the input's `value` property:

<!-- prettier-ignore -->
```svelte
<script>
	let message = $state('hello');
</script>

<input bind:value={message} />
<p>{message}</p>
```

In the case of a numeric input (`type="number"` or `type="range"`), the value will be coerced to a number ([demo](/playground/untitled#H4sIAAAAAAAAE6WPwYoCMQxAfyWEPeyiOOqx2w74Hds9pBql0IllmhGXYf5dKqwiyILsLXnwwsuI-5i4oPkaUX8yo7kCnKNQV7dNzoty4qSVBSr8jG-Poixa0KAt2z5mbb14TaxA4OCtKCm_rz4-f2m403WltrlrYhMFTtcLNkoeFGqZ8yhDF7j3CCHKzpwoDexGmqCL4jwuPUJHZ-dxVcfmyYGe5MAv-La5pbxYFf5Z9Zf_UJXb-sEMquFgJJhBmGyTW5yj8lnRaD_w9D1dAKSSj7zqAQAA)):

```svelte
<script>
	let a = $state(1);
	let b = $state(2);
</script>

<label>
	<input type="number" bind:value={a} min="0" max="10" />
	<input type="range" bind:value={a} min="0" max="10" />
</label>

<label>
	<input type="number" bind:value={b} min="0" max="10" />
	<input type="range" bind:value={b} min="0" max="10" />
</label>

<p>{a} + {b} = {a + b}</p>
```

If the input is empty or invalid (in the case of `type="number"`), the value is `undefined`.

## `<input bind:checked>`

Checkbox and radio inputs can be bound with `bind:checked`:

```svelte
<label>
	<input type="checkbox" bind:checked={accepted} />
	Accept terms and conditions
</label>
```

## `<input bind:group>`

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)" />
```

> [!NOTE] `bind:group` only works if the inputs are in the same Svelte component.

## `<input bind:files>`

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). 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`](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer) object and get `files` from there.

```svelte
<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.

> [!NOTE] `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.

## `<select bind: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>
```

## `<audio>`

`<audio>` elements have their own set of bindings — five two-way ones...

- [`currentTime`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/currentTime)
- [`playbackRate`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/playbackRate)
- [`paused`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/paused)
- [`volume`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/volume)
- [`muted`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/muted)

...and seven readonly ones:

- [`duration`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/duration)
- [`buffered`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/buffered)
- [`paused`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/paused)
- [`seekable`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/seekable)
- [`seeking`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/seeking_event)
- [`ended`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/ended)
- [`readyState`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/readyState)

```svelte
<audio src={clip} bind:duration bind:currentTime bind:paused></audio>
```

## `<video>`

`<video>` elements have all the same bindings as [#audio] elements, plus readonly [`videoWidth`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLVideoElement/videoWidth) and [`videoHeight`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLVideoElement/videoHeight) bindings.

## `<img>`

`<img>` elements have two readonly bindings:

- [`naturalWidth`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/naturalWidth)
- [`naturalHeight`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/naturalHeight)

## `<details bind:open>`

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

```svelte
<details bind:open={isOpen}>
	<summary>How do you comfort a JavaScript bug?</summary>
	<p>You console it.</p>
</details>
```

## Contenteditable bindings

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)

> [!NOTE] There are [subtle differences between `innerText` and `textContent`](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} />
```

## Dimensions

All visible elements have the following readonly bindings, measured with a `ResizeObserver`:

- [`clientWidth`](https://developer.mozilla.org/en-US/docs/Web/API/Element/clientWidth)
- [`clientHeight`](https://developer.mozilla.org/en-US/docs/Web/API/Element/clientHeight)
- [`offsetWidth`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/offsetWidth)
- [`offsetHeight`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/offsetHeight)

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

> [!NOTE] `display: inline` elements do not have a width or height (except for elements with 'intrinsic' dimensions, like `<img>` and `<canvas>`), and cannot be observed with a `ResizeObserver`. You will need to change the `display` style of these elements to something else, such as `inline-block`.

## bind:this

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

To get a reference to a DOM node, use `bind:this`. The value will be `undefined` until the component is mounted — in other words, you should read it inside an effect or an event handler, but not during component initialisation:

```svelte
<script>
	/** @type {HTMLCanvasElement} */
	let canvas;

	$effect(() => {
		const ctx = canvas.getContext('2d');
		drawStuff(ctx);
	});
</script>

<canvas bind:this={canvas} />
```

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

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

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

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

## 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 [`$bindable`]($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.
