on hover event

element.onmouseover = function() {
  //Hovering
}

<img onmouseover="enlargeImage(this)" border="0" src="smiley.gif" alt="Smiley" width="32" height="32">

<script>
function enlargeImage(x) {
  x.style.height = "64px";
  x.style.width = "64px";
}
</script>

onmouseover event 

mouse over is hover