<!DOCTYPE html>
<html>
<body>
<h2 id="demo"></h2>
<script>
var w = window.innerWidth
|| document.documentElement.clientWidth
|| document.body.clientWidth;
document.onmousemove = function(e) {
var x = (!e) ? event.clientX : e.clientX;
if (w/2 < x) {document.getElementById("demo").innerHTML = "Derecha >";}
if (w/2 > x) {document.getElementById("demo").innerHTML = "Izquierda <";}
}
</script>
</body>
</html>