<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
*{
margin: 0;
padding: 0;
}
div{width:100%}
#d1{background:#00F;height:700px;border:1px solid}
#d2{background:#FF0;height:500px;border:1px solid}
#d3{background:#F0F;height:300px;border:1px solid}
#d4{background:#F00;height:600px;border:1px solid}
</style>
<script>
window.addEventListener("DOMContentLoaded", function() {
var b = document.querySelectorAll("div");
document.addEventListener("keydown", function(e) {
for (var i=0; i < b.length; i++) {
if (window.pageYOffset < (b[i].getBoundingClientRect().top + window.pageYOffset)) break;
}
alert(i);
});
});
</script>
</head>
<body>
<div id="d1" >1↑ ↓ </div>
<div id="d2" >2</div>
<div id="d3" >3</div>
<div id="d4" >4</div>
</body>
</html>