How To ADD Commas To The String?

Using Script It Will Add Commas To The Given String....

function AddComma(MyString)
{
var objRegex = new RegExp('(-?[0-9]+)([0-9]{3})');

//Check For Criteria....
while(objRegex.test(MyString))
{
//Add First String & The Add Comma To After The First String....
MyString = MyString.replace(objRegex, '$1,$2');
}
return strValue;
}