DOM event handlers can have modifiers that alter their behaviour. For example, a handler with a once modifier will only run a single time:

<script>
	function handleClick() {
		alert('no more alerts');
	}
</script>

<button on:click|once={handleClick}> Click me </button>

The full list of modifiers:

• preventDefault — calls event.preventDefault() before running the handler. Useful for client-side form handling, for example.
• stopPropagation — calls event.stopPropagation(), preventing the event reaching the next element
• passive — improves scrolling performance on touch/wheel events (Svelte will add it automatically where it's safe to do so)
• nonpassive — explicitly set passive: false
• capture — fires the handler during the capture phase instead of the bubbling phase (MDN docs)
• once — remove the handler after the first time it runs
• self — only trigger handler if event.target is the element itself
• trusted — only trigger handler if event.isTrusted is true. I.e. if the event is triggered by a user action.

You can chain modifiers together, e.g. on:click|once|capture={...}.