index.html

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Learning Vue</title>
        <script src="https://unpkg.com/vue@3"></script>
    </head>
    <body>
        <div id="app">
            <p>
                <input type="text" v-model="greeting">
            </p>
            <p> {{ greeting }} ({{ greeting.length}}) </p>
        </div>
        <script>
            Vue.createApp({
                data() {
                    return {
                        greeting: 'Hello World'
                    }
                },

                mounted() {
                    setTimeout(() => {
                        this.greeting = 'this message was set using mounted'
                    }, 3000)
                }
            }).mount('#app');
        </script>
    </body>
</html>