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Listening to DOM events

In Svelte you can define a listener for a DOM event directly in the template, using the on:<event> syntax.

For example, to listen to the click event, you will pass a function to the on:click attribute.

To listen to the onmousemove event, you’ll pass a function to the on:mousemove attribute.

Here’s an example with the handling function defined inline:

<button on:click={() => {
  alert('clicked')
}}>Click me</button>

and here’s another example with the handling function defined in the script section of the component:

<script>
const doSomething = () => {
  alert('clicked')
}
</script>

<button on:click={doSomething}>Click me</button>

I prefer inline when the code is not too verbose. If it’s just 2-3 lines, for example, otherwise I’d move that up in the script section.

Svelte passes the event handler as the argument of the function, which is handy if you need to stop propagation or to reference something in the Event object:

<script>
const doSomething = event => {
  console.log(event)
  alert('clicked')
}
</script>

<button on:click={doSomething}>Click me</button>

Now, I mentioned “stop propagation”. That’s a very common thing to do, to stop form submit events for example. Svelte provides us modifiers, a way to apply it directly without manually doing it. stopPropagation and preventDefault are the 2 modifiers you’ll use the most, I think.

You apply a modifier like this: <button on:click|stopPropagation|preventDefault={doSomething}>Click me</button>

There are other modifiers, which are more niche. capture enables capturing events instead of bubbling, once only fires the event once, self only fires the event if the target of the event is this object (removing it from the bubbling/capturing hierarchy).

Creating your events in components

What’s interesting is that we can create custom events in components, and use the same syntax of built-in DOM events.

To do so, we must import the createEventDispatcher function from the svelte package and call it to get an event dispatcher:

<script>
  import { createEventDispatcher } from 'svelte'
  const dispatch = createEventDispatcher()
</script>

Once we do so, we can call the dispatch() function, passing a string that identifies the event (which we’ll use for the on: syntax in other components that use this):

<script>
  import { createEventDispatcher } from 'svelte'
  const dispatch = createEventDispatcher()

  //when it's time to trigger the event
  dispatch('eventName')
</script>

Now other components can use ours using

<ComponentName on:eventName={event => {
  //do something
}} />

You can also pass an object to the event, passing a second parameter to dispatch():

<script>
  import { createEventDispatcher } from 'svelte'
  const dispatch = createEventDispatcher()
  const value = 'something'

  //when it's time to trigger the event
  dispatch('eventName', value)

  //or

  dispatch('eventName', {
    someProperty: value
  })
</script>

the object passed by dispatch() is available on the event object.

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