Edit in JSFiddle

new Vue({
  el: '#app',
  data: {
    title: 'Hello World'
  },
  methods: {
    sayGoodbye: function() {
      return 'Goodbye';
    }
  }
});
<script src="https://unpkg.com/vue/dist/vue.js"></script>

<div id='app'>
  <p>{{ title }}</p>
  <p>{{ sayGoodbye() }}</p>
</div>