js+css实现全屏侧边栏

<div id="mySidenav" class="sidenav">
 <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>
 <a href="#">About</a>
 <a href="#">Services</a>
 <a href="#">Clients</a>
 <a href="#">Contact</a>
 </div>
 <h2>全屏幕侧边栏</h2>
 <p>点击以下菜单图标打开侧边栏</p>
 <span style="font-size:30px;cursor:pointer" onclick="openNav()">&#9776; 打开</span>

body {
 font-family: "Lato", sans-serif;
 }
 .sidenav {
 height: 100%;width: 0;position: fixed;z-index: 1;  top: 0; left: 0; background-color: #111; overflow-x: hidden;  transition: 0.5s;  padding-top: 60px; text-align:center;
 }
 .sidenav a {
  padding: 8px 8px 8px 32px; text-decoration: none; font-size: 25px;color: #818181; display: block; transition: 0.3s;
  }
  .sidenav a:hover{ color: #f1f1f1;
  }
  .sidenav .closebtn {position: absolute; top: 0; right: 25px; font-size: 36px; margin-left: 50px;
  }
  @media screen and (max-height: 450px) 
  { .sidenav {padding-top: 15px;
  } 
  .sidenav a {font-size: 18px;
  }
  }

function openNav() {
document.getElementById("mySidenav").style.width = "100%";
}
function closeNav() {
document.getElementById("mySidenav").style.width = "0";
}