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:

<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:

<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):

<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:

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

<input bind:group>

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.

<input bind:files>

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.

<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).

<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>

<audio>

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

• currentTime
• playbackRate
• paused
• volume
• muted

...and seven readonly ones:

• duration
• buffered
• paused
• seekable
• seeking
• ended
• readyState

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

<video>

<video> elements have all the same bindings as [#audio] elements, plus readonly videoWidth and videoHeight bindings.

<img>

<img> elements have two readonly bindings:

• naturalWidth
• naturalHeight

<details bind:open>

<details> elements support binding to the open property.

<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
• innerText
• textContent

There are subtle differences between innerText and textContent.

<div contenteditable="true" bind:innerHTML={html} />

Dimensions

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

• clientWidth
• clientHeight
• offsetWidth
• offsetHeight

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

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

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:

<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.

<ShoppingCart bind:this={cart} />

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

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

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.