How to I change this css psuedo button in Javascript/Jquery

I found this really nice checkbox button style (link below) and have been trying to turn it on and off using jQuery. I haven't used many pseudo-classes before, normally I just add an 'active' class.

https://proto.io/freebies/onoff/

I need the button to be turned ON or OFF upon page load based on server response, but I can't seem to manipulate the CSS without breaking it. How am I meant to activate/deactivate this button?

.onoffswitch {
  position: relative;
  width: 90px;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
}
.onoffswitch-checkbox {
  display: none;
}
.onoffswitch-label {
  display: block;
  overflow: hidden;
  cursor: pointer;
  border: 2px solid #999999;
  border-radius: 20px;
}
.onoffswitch-inner {
  display: block;
  width: 200%;
  margin-left: -100%;
  transition: margin 0.3s ease-in 0s;
}
.onoffswitch-inner:before,
.onoffswitch-inner:after {
  display: block;
  float: left;
  width: 50%;
  height: 30px;
  padding: 0;
  line-height: 30px;
  font-size: 14px;
  color: white;
  font-family: Trebuchet, Arial, sans-serif;
  font-weight: bold;
  box-sizing: border-box;
}
.onoffswitch-inner:before {
  content: "ON";
  padding-left: 10px;
  background-color: #34A7C1;
  color: #FFFFFF;
}
.onoffswitch-inner:after {
  content: "OFF";
  padding-right: 10px;
  background-color: #EEEEEE;
  color: #999999;
  text-align: right;
}
.onoffswitch-switch {
  display: block;
  width: 18px;
  margin: 6px;
  background: #FFFFFF;
  position: absolute;
  top: 0;
  bottom: 0;
  right: 56px;
  border: 2px solid #999999;
  border-radius: 20px;
  transition: all 0.3s ease-in 0s;
}
.onoffswitch-checkbox:checked + .onoffswitch-label .onoffswitch-inner {
  margin-left: 0;
}
.onoffswitch-checkbox:checked + .onoffswitch-label .onoffswitch-switch {
  right: 0px;
}

<div class="onoffswitch">
  <input type="checkbox" name="onoffswitch" class="onoffswitch-checkbox" id="myonoffswitch" checked>
  <label class="onoffswitch-label" for="myonoffswitch">
    <span class="onoffswitch-inner"></span>
    <span class="onoffswitch-switch"></span>
  </label>
</div>

Solution

you programmatically want to change the state, don't fiddle with CSS classes,

Just a oneliner by use of jQuery:

 $('#myonoffswitch').attr('checked', false); // turn off
 $('#myonoffswitch').attr('checked', true); // turn on

http://jsfiddle.net/5eoujsm4/

animated update: http://jsfiddle.net/5eoujsm4/1/