sapper/test/app/routes/delete-test.html

<button class='del' on:click='del()'>delete</button>

<script>
	export default {
		methods: {
			del() {
				fetch(`/api/delete/42`, { method: 'DELETE' })
					.then(r => r.json())
					.then(data => {
						window.deleted = data;
					});
			}
		}
	};
</script>