js怎样改变div的宽度

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>
<style>
#div1{width:400px;height:400px;background-color:blue;}
</style>


<body>
<div id='div1'>


</div>
<input type='button' value='big' onclick='big();' /><br/>
<input type='button' value='bigger' onclick='bigger();' />
<script>
function big(){
    var div1=document.getElementById('div1');
    div1.style.width=800;
    
}
function bigger(){
    var div1=document.getElementById('div1');
    div1.style.width=100;

var width=div1.style.width || div1.clientWidth || div1.offsetWidth || div1.scrollWidth;

/*实际用div1.offsetWidth就够了，上边的麻烦了*/
alert(width);

 
} 

</script> 

</body>

 </html>