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/* Set the width of the side navigation to 250px and the left margin of the page content to 250px and add a black background color to body */
function openNav() {
    document.getElementById("mySidenav").style.width = "250px";
    document.getElementById("myCanvasNav").style.width = "100%";
    document.getElementById("myCanvasNav").style.opacity = "0.8";  
}

/* Set the width of the side navigation to 0 and the left margin of the page content to 0, and the background color of body to white */
function closeNav() {
    document.getElementById("mySidenav").style.width = "0";
    document.getElementById("myCanvasNav").style.width = "0%";
    document.getElementById("myCanvasNav").style.opacity = "0"; 
}

 <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>

<!-- Use any element to open the sidenav -->
<span onclick="openNav()">open</span>
<div id="myCanvasNav" class="menu-overlay"" onclick="closeNav()"></div>

/* The side navigation menu */
.sidenav {
    height: 100%; /* 100% Full-height */
    width: 0; /* 0 width - change this with JavaScript */
    position: fixed; /* Stay in place */
    z-index: 1; /* Stay on top */
    top: 0;
    left: 0;
    background-color: #111; /* Black*/
    overflow-x: hidden; /* Disable horizontal scroll */
    padding-top: 60px; /* Place content 60px from the top */
    transition: 0.5s; /* 0.5 second transition effect to slide in the sidenav */
    z-index: 999;
}

/* The navigation menu links */
.sidenav a {
    padding: 8px 8px 8px 32px;
    text-decoration: none;
    font-size: 25px;
    color: #818181;
    display: block;
    transition: 0.3s
}

/* When you mouse over the navigation links, change their color */
.sidenav a:hover, .offcanvas a:focus{
    color: #f1f1f1;
}

/* Position and style the close button (top right corner) */
.sidenav .closebtn {
    position: absolute;
    top: 0;
    right: 25px;
    font-size: 36px;
    margin-left: 50px;
}

/* Style page content - use this if you want to push the page content to the right when you open the side navigation */
#main {
    transition: margin-left .5s;
    padding: 20px;
}

/* On smaller screens, where height is less than 450px, change the style of the sidenav (less padding and a smaller font size) */
@media screen and (max-height: 450px) {
    .sidenav {padding-top: 15px;}
    .sidenav a {font-size: 18px;}
}

.menu-overlay {
  height: 100%;
  width: 0;
  position: fixed;
  z-index: 3;
  top: 0;
  left: 0;
  background-color: rgba(0,0,0, 0.9);
  overflow-y: auto;
  overflow-x: hidden;
  text-align: center;
  opacity: 0;
  transition: opacity 1s;
}