```
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script src="https://cdn.bootcss.com/vue/2.5.18-beta.0/vue.js"></script>
</head>
<body>
<div id="app">
{{sum}}
</div>
<script>
new Vue({
el: "#app",
data: {
num1: 10,
num2: 20,
sum: 30
},
// 監聽data里面的某個值改變的時候觸發
watch:{
num1(){
this.sum = this.num1 + this.num2
},
num2:function(){
this.sum = this.num1 + this.num2
}
}
})
</script>
</body>
</html>
```
