为什么我没输入内容的时候，fullName所在div显示undefined，vue版本2.6.10
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<script src="js/vue.js"></script>
</head>
<body>
<div id="root">
姓：<input type="text" v-model="firstName"/>
名：<input type="text" v-model="lastName"/>
<div>{{fullName}}</div>
</div>
<script type="text/javascript">
new Vue({
el:"#root",
data:{
firsrName:'',
lastName:'',
},
computed :{
fullName:function(){
return this.firstName +this.lastName
}
}
})
</script>
</body>
</html>
