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

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

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

		oncreate () {
			this.observe( 'foo', foo => {
				this.set({ bar: foo * 2 });
			});
		}
	};
</script>