index.html

<!doctype html>
<html lang="zh-Hans">
<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">
    <title>Document</title>
</head>
<body>
<div id="app">
   <g-button></g-button>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script src="./button.js"></script>
<script>
new Vue({
    el:'#app',
    data:{
        message:'hi'
    }
})
</script>
</body>
</html>

button.js

Vue.component('g-button',{
    template:`
    <button>hi</button>>
    `
})

运行结果