<div id="app">
  <p>Full name: {{ fullName }}</p>
  <input v-model="firstName" placeholder="First name" />
  <input v-model="lastName" placeholder="Last name" />
</div>

<script src="https://unpkg.com/vue@3/dist/vue.global.js"></script>
<script>
  const app = Vue.createApp({
    data() {
      return {
        firstName: 'Ada',
        lastName: 'Lovelace'
      }
    },
    computed: {
      fullName() {
        return this.firstName + ' ' + this.lastName
      }
    }
  })
  app.mount('#app')
</script>