CSSとJavaScriptを使用して全画面検索ボックスを作成するにはどうすればよいですか？

以下は、CSSとJavaScriptを使用して全画面検索ボックスを作成するためのコードです-

例

<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/fontawesome.min.css">
<style>
body {
   font-family: Arial;
}
* {
   box-sizing: border-box;
}
.showBtn {
   background: #008b0c;
   border: none;
   color:white;
   padding: 10px 15px;
   font-size: 20px;
   cursor: pointer;
   opacity: 0.8;
}
.showBtn:hover {
   opacity: 1;
}
.overlaySearch {
   height: 100%;
   width: 100%;
   display: none;
   position: fixed;
   z-index: 1;
   top: 0;
   left: 0;
   background-color: rgba(132, 150, 155, 0.747);
}
.searchBar {
   position: relative;
   top: 46%;
   width: 80%;
   text-align: center;
   margin-top: 30px;
   margin: auto;
}
.overlaySearch .hideBtn {
   position: absolute;
   top: 20px;
   right: 45px;
   font-size: 60px;
   cursor: pointer;
   color: rgb(255, 0, 0);
   opacity: 0.8;
}
.overlaySearch .hideBtn:hover {
   opacity: 1;
}
.overlaySearch input[type=text] {
   padding: 15px;
   font-size: 17px;
   border: none;
   float: left;
   width: 80%;
   background: white;
}
.overlaySearch input[type=text]:hover {
   background: #f1f1f1;
}
.overlaySearch button {
   float: left;
   width: 20%;
   padding: 15px;
   background: rgb(54, 21, 241);
   font-size: 17px;
   border: none;
   color:white;
   cursor: pointer;
   opacity: 0.8;
}
.overlaySearch button:hover {
   opacity: 1;
}
</style>
</head>
<body>
<div class="overlaySearch" >
<span class="hideBtn">×</span>
<div class="searchBar">
<form >
<input type="text" placeholder="Search Here.." ">
<button type="submit"><i class="fa fa-search"></i></button>
</form>
</div>
</div>
<h1>Fullscreen Search Example</h2>
<button class="showBtn">Open Search Box</button>
<h2>Click on the above button to open search box in full screen</h2>
<script>
document.querySelector('.hideBtn').addEventListener('click',hideSearch);
document.querySelector('.showBtn').addEventListener('click',showSearch);
function showSearch() {
   document.querySelector('.overlaySearch').style.display = "block";
}
function hideSearch() {
   document.querySelector('.overlaySearch').style.display = "none";
}
</script>
</body>
</html>