简单问题，数组排序

[code=JScript]<!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>
    <title>数据对象</title>
    <script type="text/javascript" language="javascript">
        function isGreater(a,b)
        {
            return Math.abs(a) - Math.abs(b);
        }
    </script>
</head>
<body>
    <script type="text/javascript" language="javascript">
        numericalOrder=new Array(-3,-2,-1,0,1,2,3);
        document.write("before sort:<br>");
        document.write(numericalOrder.join("/"));
        document.write("<br>");
        document.write("after sort:<br>");
        
        //自定义方法排序,这里无输出了？为什么
        numericalOrder.sort(isGreater)
        document.write(numericalOrder.join("/"));
    </script>
</body>
</html>
[/code]no.1 Array.sort返回值为空
no.2 if(Math.abs(a)=Math.abs(b)) return 0;
  "==" != "="