index.html

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link  type="text/css" rel="stylesheet" href="./main.css">
    <script src="https://unpkg.com/vue"></script>

    <title>Document</title>
</head>
<body>
<!--vue app是根容器-->
<div id="vue-app">
    <h1>{{age}}</h1>
<button v-on:click="add">+++</button>
    <button v-on:click="substrct">---</button>
</div>
<script src="./app.js"></script>
</body>
</html>

app.js

new Vue({
    el:"#vue-app",
    data:{
        age:18,
    },
    methods:{
           add:function (){
               this.age++
        },
        substrct:function (){
               this.age--
        }
    }
})