[danik.js]

<div style="height:150px;"></div>
 
<script>
    var div = document.querySelector('div');
    div.addEventListener('mousemove', function(event) {
        if (event.offsetX > div.offsetWidth / 2)
            div.style.backgroundColor = 'black';
        else
            div.style.backgroundColor = 'red';
    });
</script>