How to create a full-screen overlay navigation menu using javascript

We click on the element to open the fullscreen overlay navigation menu. The navigation menu will slide in from left to right, if we click on the close icon navigation will be closed 

Step 1) Add HTML:

<!-- The overlay -->
<div id="myNav" class="overlay">

  <!-- Button to close the overlay navigation -->
  <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>

  <!-- Overlay content -->
  <div class="overlay-content">
    <a href="#">About</a>
    <a href="#">Services</a>
    <a href="#">Clients</a>
    <a href="#">Contact</a>
  </div>

</div>

<!-- Use any element to open/show the overlay navigation menu -->
<span onclick="openNav()">open</span>

Step 2) Add CSS:

/* The Overlay (background) */
.overlay {
  /* Height & width depends on how you want to reveal the overlay (see JS below) */   
  height: 100%;
  width: 0;
  position: fixed; /* Stay in place */
  z-index: 1; /* Sit on top */
  left: 0;
  top: 0;
  background-color: rgb(0,0,0); /* Black fallback color */
  background-color: rgba(0,0,0, 0.9); /* Black w/opacity */
  overflow-x: hidden; /* Disable horizontal scroll */
  transition: 0.5s; /* 0.5 second transition effect to slide in or slide down the overlay (height or width, depending on reveal) */
}

/* Position the content inside the overlay */
.overlay-content {
  position: relative;
  top: 25%; /* 25% from the top */
  width: 100%; /* 100% width */
  text-align: center; /* Centered text/links */
  margin-top: 30px; /* 30px top margin to avoid conflict with the close button on smaller screens */
}

/* The navigation links inside the overlay */
.overlay a {
  padding: 8px;
  text-decoration: none;
  font-size: 36px;
  color: #818181;
  display: block; /* Display block instead of inline */
  transition: 0.3s; /* Transition effects on hover (color) */
}

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

/* Position the close button (top right corner) */
.overlay .closebtn {
  position: absolute;
  top: 20px;
  right: 45px;
  font-size: 60px;
}

/* When the height of the screen is less than 450 pixels, change the font-size of the links and position the close button again, so they don't overlap */
@media screen and (max-height: 450px) {
  .overlay a {font-size: 20px}
  .overlay .closebtn {
    font-size: 40px;
    top: 15px;
    right: 35px;
  }
}

Step 3) Add JavaScript:

The example below slides in the overlay navigation menu from left to right (0 to 100% width), when it is triggered

/* Open when someone clicks on the span element */
function openNav() {
  document.getElementById("myNav").style.width = "100%";
}

/* Close when someone clicks on the "x" symbol inside the overlay */
function closeNav() {
  document.getElementById("myNav").style.width = "0%";
}