svelte/documentation/examples/03-logic/00-if-blocks/App.svelte

<script>
	let user = { loggedIn: false };

	function toggle() {
		user.loggedIn = !user.loggedIn;
	}
</script>

{#if user.loggedIn}
	<button on:click={toggle}> Log out </button>
{/if}

{#if !user.loggedIn}
	<button on:click={toggle}> Log in </button>
{/if}