不知道哪里错了，输出不了,帮忙看看
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>二维数组</title>
<script type="text/javascript">
var mar=new Array();
for(var a=0;a<3;a++)
{
mar[a]=new Array();
for(var b=0;b<6;b++)
{
mar[a][b]=a*b;
}
}
document.write(mar[a][b]);
</script>
</head>
<body>
</body>
</html>
