<input type="button" value="a(2,3)" onclick="a(2,3);" />
<script type="text/javascript">
<!--
function a(val1){
    alert(arguments.length);
}
//-->
</script>

関数名.arguments[n]

<input type="button" value="b(2,3)" onclick="b(2,3);" />
<script type="text/javascript">
<!--
function b(val1){
    alert("b.arguments[0]=" + b.arguments[0]);
    alert("b.arguments[1]=" + b.arguments[1]);
    alert("b.arguments[2]=" + b.arguments[2]);
    
    for(i in arguments){
        alert(arguments[i]);
    }
}
//-->
</script>