1element.addEventListener('mouseover',someFunction);

1element.onmouseover = function() {
2  //Hovering
3}

1<img onmouseover="enlargeImage(this)" border="0" src="smiley.gif" alt="Smiley" width="32" height="32">
2
3<script>
4function enlargeImage(x) {
5  x.style.height = "64px";
6  x.style.width = "64px";
7}
8</script>

1onmouseover event