---
title: Component directives
---

## on:_eventname_

```svelte
<!--- copy: false --->
on:eventname={handler}
```

Components can emit events using [`createEventDispatcher`](/docs/svelte#createeventdispatcher) or by forwarding DOM events.

```svelte
<script>
	import { createEventDispatcher } from 'svelte';

	const dispatch = createEventDispatcher();
</script>

<!-- programmatic dispatching -->
<button on:click={() => dispatch('hello')}> one </button>

<!-- declarative event forwarding -->
<button on:click> two </button>
```

Listening for component events looks the same as listening for DOM events:

```svelte
<SomeComponent on:whatever={handler} />
```

As with DOM events, if the `on:` directive is used without a value, the event will be forwarded, meaning that a consumer can listen for it.

```svelte
<SomeComponent on:whatever />
```

## --style-props

```svelte
<!--- copy: false --->
--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:

```svelte
<Slider bind:value min={0} --rail-color="black" --track-color="rgb(0, 0, 255)" />
```

Desugars to this:

```svelte
<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:

```svelte
<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:

```svelte
<style>
	.potato-slider-rail {
		background-color: var(--rail-color, var(--theme-color, 'purple'));
	}
</style>
```

So you can set a high-level theme color:

```css
/* global.css */
html {
	--theme-color: black;
}
```

Or override it at the consumer level:

```svelte
<Slider --rail-color="goldenrod" />
```

## bind:_property_

```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.

## bind:this

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

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

```svelte
<ShoppingCart bind:this={cart} />

<button on:click={() => cart.empty()}> Empty shopping cart </button>
```

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