define var vuejs
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Vue.js Example</title>
</head>
<body>
<div id="app">
{{ message }}
</div>
<script src="https://cdn.jsdelivr.net/npm/vue@2"></script>
<script>
var app = new Vue({
el: '#app',
data: {
message: 'Hello, Vue!'
}
});
</script>
</body>
</html>