Vue

HTML

<script src="https://cdn.jsdelivr.net/npm/@vue/composition-api@1.0.0-rc.8"></script>
<div id="app">
  <h2 @click="ctx.fn">{{ ctx.counter }} {{counter}} </h2>
</div>

Vue

const { reactive, toRefs } = VueCompositionAPI;

const useCounter = () => {
  const ctx = reactive({
    counter: 2,
   	// ...
  });
  
  const fn = () => {
  	console.log('Called', ctx.counter);
  }
    
  setInterval(() => ctx.counter += 1, 1000);
  return {...toRefs(ctx), fn};
}

new Vue({
  el: "#app",
  setup() {
    const ctx = useCounter();
    const { counter } = useCounter();
    return { ctx, counter };
  }
})