Class Attribute

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