Elastic Sidebar Navigation With CSS3 And JavaScript

A fancy sidebar off-canvas navigation menu that appears with an elastic animation effect based on CSS3 and a little bit of JavaScript.

How to use it:

Create the sidebar navigation.

<div id="nav" class="navigation">
  <div class="navigation__inner">
    <!-- Navigation Content Here -->
  </div>
</div>

Create a trigger button to open/close the sidebar navigation.

<button id="show">Toggle Menu</button>

The primary CSS/CSS3 styles.

.navigation {
  position: fixed;
  width: 300px;
  height: 100%;
  top: 0;
  overflow-y: auto;
  overflow-x: hidden;
  opacity: 0;
  visibility: hidden;
  z-index: 99;
  -webkit-transition-delay: 300ms;
  transition-delay: 300ms;
  left: 0;
}
.navigation.active {
  opacity: 1;
  visibility: visible;
  -webkit-transition-delay: 0s;
  transition-delay: 0s;
}
.navigation.active .navigation__inner {
  background-color: #7c7fe0;
  -webkit-transform: translate(0, 0);
  transform: translate(0, 0);
  -webkit-transition: background-color 0s linear 599ms, -webkit-transform 300ms linear;
  transition: background-color 0s linear 599ms, -webkit-transform 300ms linear;
  transition: transform 300ms linear, background-color 0s linear 599ms;
  transition: transform 300ms linear, background-color 0s linear 599ms, -webkit-transform 300ms linear;
}
.navigation.active .navigation__inner:after {
  width: 300%;
  border-radius: 50%;
  -webkit-animation: elastic 150ms ease 300.5ms both;
  animation: elastic 150ms ease 300.5ms both;
}
.navigation__inner {
  position: absolute;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  overflow: hidden;
  z-index: 999999;
  -webkit-transform: translate(-100%, 0);
  transform: translate(-100%, 0);
  -webkit-transition: background-color 0s linear 300ms, -webkit-transform 300ms linear;
  transition: background-color 0s linear 300ms, -webkit-transform 300ms linear;
  transition: transform 300ms linear, background-color 0s linear 300ms;
  transition: transform 300ms linear, background-color 0s linear 300ms, -webkit-transform 300ms linear;
}
.navigation__inner:after {
  content: '';
  position: absolute;
  width: 0;
  height: 100%;
  top: 0;
  right: 0;
  background-color: #7c7fe0;
  border-radius: 50%;
  z-index: -1;
  -webkit-transition: all 300ms linear;
  transition: all 300ms linear;
}

Create the elastic effect in the CSS3 keyframes.

@-webkit-keyframes 
elastic {  0% {
 border-radius: 50%;
}
 45% {
 border-radius: 0;
}
 65% {
 border-top-right-radius: 40px 50%;
 border-bottom-right-radius: 40px 50%;
}
 80% {
 border-radius: 0;
}
 90% {
 border-top-right-radius: 20px 50%;
 border-bottom-right-radius: 20px 50%;
}
 100% {
 border-radius: 0;
}
}
@keyframes 
elastic {  0% {
 border-radius: 50%;
}
 45% {
 border-radius: 0;
}
 65% {
 border-top-right-radius: 40px 50%;
 border-bottom-right-radius: 40px 50%;
}
 80% {
 border-radius: 0;
}
 90% {
 border-top-right-radius: 20px 50%;
 border-bottom-right-radius: 20px 50%;
}
 100% {
 border-radius: 0;
}
}

Use plain JavaScript to toggle CSS classes depending on the current status.

var btn = document.getElementById('show');
var nav = document.getElementById('nav');
btn.addEventListener('click', function() {
    nav.classList.toggle('active');
});