Thanks @Jeffrey for refreshing from Vue 2 to Vue 3.
<!doctype html>
<html >
<head>
    <title>Vue 3 Absolute Basics</title>
    <script src="https://unpkg.com/vue@3"></script>
</head>
<body>
    <div id="app">
        
        <input type="text" v-model="greeting">

        <p>
            {{ greeting }} ({{ greeting.length }})
        </p>
    </div>
    <script>
        Vue.createApp({
            data() {
                return {
                    greeting: 'Hello world !'
                };
            },
            
            mounted(){
                setTimeout(()=>{
                    this.greeting = 'changed !';
                }, 3000);
            }

        }).mount('#app');
    </script>
</body>
</html>