错误信息：Property "msg" was accessed during render but is not defined on instance.
代码，为什么会报错呀？
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<div id="app">
<h1>{{msg}}</h1>
</div>
<script type="module" >
import { createApp } from "https://unpkg.com/vue@3/dist/vue.esm-browser.js"
createApp({
date() {
msg:'hello';
return {
msg
}
}
}).mount("#app")
</script>
</body>
</html>
