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$(function(){
$(window).scroll(function() {
if($(window).scrollTop() >= 100) {
$("#header").addClass("menu_bg");
}else{
var current = $("#header ");
current.removeClass("menu_bg");
current = $(this);
}
})
}
);
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script src="jquery-3.1.1.min.js"></script>
<style>
body{
height:2000px;
margin:0
}
div{
background:red;
height:200px;
width:100%;
position:fixed;
}
</style>
<script>
$(window).scroll(function(){
$('div').css({
opacity:(2000-($('div').offset().top))/2000
});
});
</script>
</head>
<body>
<div></div>
</body>
</html>