you can use toggleClass()
to toggle class it is really handy.
case:1
<div id='mydiv' class="class1"></div>
$('#mydiv').toggleClass('class1 class2');
output: <div id='mydiv' class="class2"></div>
case:2
<div id='mydiv' class="class2"></div>
$('#mydiv').toggleClass('class1 class2');
output: <div id='mydiv' class="class1"></div>
case:3
<div id='mydiv' class="class1 class2 class3"></div>
$('#mydiv').toggleClass('class1 class2');
output: <div id='mydiv' class="class3"></div>