Class Attribute

// Reads the first and last name of each person and adds a new attribute with the full name.
var xml = '<roster>'
    + '<person first="John" last="Doe"/>'
    + '<person first="Mary" last="Smith"/>'
    + '</roster>';
var document = XmlService.parse(xml);
var people = document.getRootElement().getChildren('person');
for (var i = 0; i < people.length; i++) {
  var person = people[i];
  var firstName = person.getAttribute('first').getValue();
  var lastName = person.getAttribute('last').getValue();
  person.setAttribute('full', firstName + ' ' + lastName);
}
xml = XmlService.getPrettyFormat().format(document);
Logger.log(xml);