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svelte/documentation/docs/02-template-syntax/06-component-directives.md

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---
title: Component directives
---
## on:_eventname_
```svelte
on:eventname={handler}
```
Components can emit events using [`createEventDispatcher`](/docs/svelte#createeventdispatcher) or by forwarding DOM events.
```svelte
<!-- SomeComponent.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
--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
<!-- Slider.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
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.