<div id="app">
  <input type="number" placeholder="enter a number" v-model="n">
  <button type="button" @click.prevent="addToSet()">Add to Set</button>
</div>

new Vue({
  el: "#app",
  data: {
    n: null,
    nSet: new Set()
  },
  methods: {
    addToSet: function() {
      this.nSet.add(this.n);
      console.log(this.n + ' added to Set');
    }
  },
  watch: {
    nSet: function (newVal, oldVal) {
      console.log(newVal);
    }
  }
});