svelte/test/runtime/samples/set-in-onstate/main.html

<p>{{foo}}</p>
<p>{{bar}}</p>

<script>
	export default {
		data: () => ({
			foo: 1
		}),

		onstate({ changed, current }) {
			if (changed.foo) {
				this.set({ bar: current.foo * 2 });
			}
		}
	};
</script>