import CmmnModdle from 'cmmn-moddle';
var moddle = new CmmnModdle();
var xmlStr =
  '<?xml version="1.0" encoding="UTF-8"?>' +
  '<cmmn:definitions xmlns:cmmn="https://www.omg.org/spec/CMMN/20150516/MODEL" id="empty-definitions" targetNamespace="https://bpmn.io/schema/cmmn">' +
  '</cmmn:definitions>';
moddle.fromXML(xmlStr, function(err, definitions) {
  // update id attribute
  definitions.set('id', 'NEW ID');
  // add a root element
  var cmmnCase = moddle.create('cmmn:Case', { id: 'MyCase_1' });
  definitions.get('cases').push(cmmnCase);
  moddle.toXML(definitions, function(err, xmlStrUpdated) {
    // xmlStrUpdated contains new id and the added process
  });
});