clear the content of _lastAlert()

hemohini

I have written below mentioned code but I want to clear the content of last alert.
Please help

function navigateToGroup($GroupName, $Description)
{

//Login to the Plant Connect
_setValue(_textbox("loginname"), "SuperUser");
_setValue(_password("password"), "SuperUser");
_click(_submit("Submit"));
_click(_image("admin.gif"));
_wait(3000);

//Open Site Admin>Group Page
_popup("Admin")._click(_link("Group"));
_popup("Admin")._click(_link("Add[1]"));
_popup("Admin")._setSelected(_select("groupType"), "Group1");

// Get the DB instance
var db = _getDB("sun.jdbc.odbc.JdbcOdbcDriver","jdbc:odbc:Driver={Microsoft Excel Driver (*.xls)};DBQ=D:\\Sahi_New\\scripts\\test.xls","","");
var $rs;
var $rs = db.select("select * from [Sheet1$]"); // Name of the Sheet
var $rs2;
var $rs2 = db.select("select * from [Sheet2$]"); // Name of the Sheet
var $cls = "null";

// Group Name Validation
for ($i=0; $i<$len; $i++)
{
var $GroupName =($rs[$i]["GroupName"]); // Name of the column
var $Description =($rs[$i]["Description"]); // Name of the column
if($GroupName == "null")
{
$GroupName="";
}
_wait(3000);
_popup("Admin")._setValue(_textbox("groupNm"), $GroupName);
_popup("Admin")._setValue(_textarea("description"), $Description);
_popup("Admin")._click(_submit(" Save "));
for ($j=0; $j<3; $j++)
{
var $Errors =($rs2[$j]["Errors"]); // Name of the column
_assertEqual($Errors, _lastAlert());
}

_set($cls, _lastAlert());
_popup("Admin")._setSelected(_select("groupType"), "Group1");
_wait(2000);
} //End of function
}
//Process Group Name Validation
$len=6;
var $excelTab= "[Sheet1$]";
navigateToGroup($excelTab);