数组索引对数据长度的影响？
<!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>数组长度</title>
<script language="javascript">
var mynum=new Array(65,90,88,98);
mynum[7]=77;
document.write("数组的长度是:"+ mynum.length );
</script>
</head>
<body>
</body>
</html>
结果是8, 数组里只有5个数啊，请问？
