svelte/test/runtime/samples/helpers-invoked-if-changed/main.html

<p>{{x}}</p>
<p class='{{getClass(y)}}'>{{myHelper(z)}}</p>

<script>
	import counter from './counter.js';

	export default {
		helpers: {
			getClass(value) {
				counter.y += 1;
				return value;
			},

			myHelper(value) {
				counter.z += 1;
				return value;
			}
		}
	};
</script>