I think you have to use window.onload event for this,you can use this like this..
HTML
<body onload="myFunction()">
<div id="foo">
<div id="bar">
Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world. Hello world.
</div>
</div>
</body>
JAVASCRIPT
<script type="text/javascript">
function myFunction(){
document.getElementById('foo').scrollLeft = 500;
}
</script>