svelte/test/runtime/samples/custom-method/main.html

<button on:click='add1()'>+1</button>

<p>{counter}</p>

<script>
	export default {
		data: () => ({
			counter: 0
		}),

		methods: {
			add1 () {
				this.set({ counter: this.get().counter + 1 });
			},

			foo () {
				return 42;
			}
		}
	};
</script>