<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.box{
width: 300px;
height: 300px;
display: none;
display: flex;
}
</style>
</head>
<body>
<div id="box" class="box"></div>
<script>
let dom = document.getElementById('box')
console.log(window.getComputedStyle(dom,null).getPropertyValue("display"))
</script>
</body>
</html>