我为什么网页上输出框下面是{{firstname}} {{lastname}}
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>计算属性与侦听器</title>
<script src="./vue.js"></script>
</head>
<body>
<div id="root">
学号<input v-model="firstname" />
姓名<input v-model="lastname" />
<div> {{firstname}}{{lastname}}</div>
</div>
<script>
new Vue({
el:"#root",
data:｛
firstname:'',
lastname:''
｝
})
</script>
</body>
</html>
