let xhr = new XMLHttpRequest();

xhr.open("GET", "file.xml", true);
xhr.responseType = "document";
xhr.overrideMimeType("text/xml");
xhr.onload = function () {
  if (xhr.readyState === xhr.DONE) {
    if (xhr.status === 200) {
      let programmes = xhr.response.getElementsByTagName("programme");

      for (let i = 0, t = programmes.length; i < t; ++i) {
        let categories = programmes[i].getElementsByTagName("category");

        console.log("Title: " + programmes[i].getElementsByTagName("title")[0].childNodes[0].nodeValue);

        for (let j = 0, k = categories.length; j < k; ++j) {
          console.log("Category: " + categories[j].childNodes[0].nodeValue);
        }
      }
    }
  }
};

xhr.send();