Vue Js Watch Property

<!DOCTYPE html>
<html>
<head>
    <script src="https://unpkg.com/vue@3/dist/vue.global.js"></script>
</head>
<body>
    <div id="app">
      <h3>Vue Js Watch Property </h3>
        <label>First Name:</label>
        <input type="text" v-model="firstName">

<p>Full Name: {{ fullName }}</p>

</div>
    <script type="module">
        const app = Vue.createApp({
            data() {
                return {
                    firstName: '',
                    lastName: '',
                    fullName: ''
                }
            },
            watch: {
                firstName: function (newVal, oldVal) {
                    this.fullName = this.firstName + ' ' + this.lastName;
                },
                lastName: function (newVal, oldVal) {
                    this.fullName = this.firstName + ' ' + this.lastName;
                }
            }
        });
        app.mount('#app');
    </script>
    <style scoped>
        #app {
            display: flex;
            flex-direction: column;
            align-items: center;
            justify-content: center;
            height: 100vh;
        }

label {
            margin-bottom: 0.5rem;
            font-weight: bold;
        }

input[type="text"] {
            padding: 0.5rem;
            border: 1px solid #ccc;
            border-radius: 0.25rem;
            margin-bottom: 1rem;
        }

p {
            font-weight: bold;
        }
    </style>
</body>

</html>