gecko-dev/dom/workers/test/instanceof_worker.js

/**
 * Any copyright is dedicated to the Public Domain.
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
onmessage = function(event) {
  postMessage({
    event: "XMLHttpRequest",
    status: new XMLHttpRequest() instanceof XMLHttpRequest,
    last: false,
  });
  postMessage({
    event: "XMLHttpRequestUpload",
    status: new XMLHttpRequest().upload instanceof XMLHttpRequestUpload,
    last: true,
  });
};