process.js

// let realRawData = `...`

// Note: The following code uses inconsistant processing strategies for the purposes of example.
{
  //// Utility Functions ////
  const querySelectorAll = (node, expr) => node.querySelectorAll(expr);
  const querySelector = (node, expr) => node.querySelector(expr);

  function* queryXPathAll(node, expr) {
    const evaluator = new XPathEvaluator();

    const document = node.ownerDocument?.documentElement ?? node.documentElement;
    const resolver = evaluator.createNSResolver(document);

    const results = evaluator.evaluate(expr, node, resolver, 0, null);

    let result;
    while (result = results.iterateNext()) {
      yield result;
    }
  }
  
  function queryXPath(node, expr){
    for (const result of queryXPathAll(node, expr)){
      return result;
    }
  }

  // Process NPCs found in `node`. Emit raw data (or possibly actual Actors).
  function processNPCs(node){
    let npcNodes = querySelectorAll(node, 'root>npc>*');
    for (const npcNode of npcNodes){
      yield processNPC(npcNode));
    }
  }
  
  function processNPC(npcNode){
    let npcData = {
      name: querySelector(npcNode, 'name').innerHTML,
      abilities: processAbilities(npcNode)
    }

    return npcData;
  }
  
  // Process NPC abilities found in `npcNode`. Return an array of ability data (or perhaps Items)
  function processAbilities(npcNode){
    let powerNodes = Array.from(querySelectorAll(npcNode, 'powers'))
    return powerNodes.map(processAbility);
  }
  
  function processAbility(powerNode){
    // ...
  }
  
  
  //// Main ////
  const rawData = realRawData;
  const data = new DOMParser().parseFromString(rawData, "text/xml");

  // Demonstration of XPath vs CSS Selectors
  console.log(Array.from(queryXPathAll(data, '/root/npc/*')));
  console.log(Array.from(querySelectorAll(data, 'root>npc>*')));
  
  for (const npcNode of queryXPathAll(data, '/root/npc/*')){
    console.log(processNPC(npcNode));
  }
}