// init your element
var $nav = $('div.single-level-nav');
// use a boolean var to check if the element is already shrinked
var navShrinked = false;
$(window).on('scroll', function() {
// init your element
var $nav = $('div.single-level-nav');
if ($(this).scrollTop() > 1){
// add class only once
if(!navShrinked) {
$nav.addClass("shrunk");
navShrinked = true;
}
}
else{
// remove class only once
if(navShrinked) {
$nav.removeClass("shrunk");
navShrinked = false;
}
}
});
body {
height: 1500px;
}
.single-level-nav {
height: 100px;
width: 100%;
position: fixed;
top: 0;
left: 0;
background-color: #000;
transition: height .3s;
}
.single-level-nav.shrunk {
height: 50px;
}
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title></title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js" ></script>
</head>
<body>
<div class="single-level-nav"></div>
</body>
</html>