<!DOCTYPE html>
<html lang="en" xmlns:v-bind="http://www.w3.org/1999/xhtml" xmlns:v-on="http://www.w3.org/1999/xhtml">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<div id="app">
<div>{{num}}</div>
<div><button @click="add">add</button></div>
<div><button @click="reduce">reduce</button></div>
</div>
<script src="https://cdn.bootcss.com/vue/2.5.17-beta.0/vue.js"></script>
<script>
new Vue({
el: '#app',
data: {
num:0
},
methods:{
add:function(){
this.num = this.num +1
},
reduce:function(){
if(this.num<=0){
this.num = 0;
}
else{
this.num = this.num -1
}
}
}
})
</script>
</script>
</body>
</html>