这应该行得通：

window.addEventListener("load",onLoadHandler);
function onLoadHandler(e){
  let side = document.getElementById("mySidenav",closeNav);
  side.addEventListener("click",closeNav)
}

function openNav() {
  document.getElementById("mySidenav").style.width = "250px";
}

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

body {
       font-family: "Lato", sans-serif;
     }

     .sidenav {
      height: 100%;
      width: 0;
      position: fixed;
      z-index: 1;
      top: 0;
      right: 0;
      background-color: #111;
      overflow-x: hidden;
      transition: 0.5s;
      padding-top: 60px;
      }

    .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;}
    }

<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>Right-sided Navigation</h2>
<p>Click on the element below to open the right-sided navigation menu.</p>
<span style="font-size:30px;cursor:pointer" onclick="openNav()">&#9776; open</span>

祝你愉快，尼古拉斯.