gecko-dev/dom/base/test/mozbrowser_api_utils.js

const FRAME_URL = "http://example.org/";

const METHODS = {
  sendMouseEvent: {},
  goBack: {},
  goForward: {},
  reload: {},
  stop: {},
  getCanGoBack: {},
  getCanGoForward: {},
};

const ATTRIBUTES = [];

function once(target, eventName, useCapture = false) {
  info(
    "Waiting for event: '" + JSON.stringify(eventName) + "' on " + target + "."
  );

  return new Promise(resolve => {
    for (let [add, remove] of [
      ["addEventListener", "removeEventListener"],
      ["addMessageListener", "removeMessageListener"],
    ]) {
      if (add in target && remove in target) {
        eventName.forEach(evName => {
          target[add](
            evName,
            function onEvent(...aArgs) {
              info("Got event: '" + evName + "' on " + target + ".");
              target[remove](evName, onEvent, useCapture);
              resolve(aArgs);
            },
            useCapture
          );
        });
        break;
      }
    }
  });
}

async function loadFrame(attributes = {}) {
  let iframe = document.createElement("iframe");
  iframe.setAttribute("src", FRAME_URL);
  for (let key in attributes) {
    iframe.setAttribute(key, attributes[key]);
  }
  let loaded = once(iframe, ["load", "mozbrowserloadend"]);
  document.body.appendChild(iframe);
  await loaded;
  return iframe;
}