count数值为什么不会增加

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <script src="./vue.js"></script>
</head>
<body>
    <div id="root">
        姓：<input v-model:value="firstName">
        名：<input v-model:value="lastName">
        <div>{{fullName}}</div>
        <div>{{count}}</div>
    </div>

    <script>
        new Vue({
            el:"#root",
            data:{
                firstName:"",
                lastName:"",
                count: 0
 },
            computed:{
                fullName: function () {
                    return this.firstName+" "+this.lastName
 }
            },
            watch:{
                firstName: function () {
                    this,count++
                }
            }
        })
    </script>
</body>
</html>