Finding mac address of client machine

It depends on security level page runs on. If you are running script using some elevated privileges, on a windows machine, one of code snippets below below could work

for (var arr = [], enum_ = new Enumerator(GetObject('WinMgmts:\\\\.\\Root\\CIMV2').ExecQuery('SELECT * FROM Win32_NetworkAdapter', 'WQL', 0x10 | 0x20)); !enum_.atEnd(); enum_.moveNext()) {
    var mac = enum_.item().MACAddress;
    mac && arr.push(mac);
}
WScript.Echo(arr.join('\n'));

var arr = [], WScriptExec = WScript.CreateObject('WScript.Shell').Exec('ipconfig /all');
while (!WScriptExec.status) WScript.Sleep(100);
WScript.Echo(WScriptExec.StdOut.ReadAll().match(/(?:[\da-f]{2}\-){5}[\da-f]{2}/gi).join('\n'));

From the web page, unless you use some undocumented feature or specific component, it would not be possible to get the MAC address.