wait for blocked or loaded image

blocked_image_promise_snippet.js

let blockedImagePromise = new Promise((resolve, reject) => {
  let blockedImage = document.createElement("img");
  blockedImage.setAttribute("src", "http://example.com/tests/toolkit/components/extensions/test/mochitest/file_image_bad.png");
  let errorListener = () => {
    browser.test.succeed("img got blocked as expected"); 
    cleanupListeners();
    resolve();
  };
  let loadListener = () => {
    browser.test.fail("img loaded unexpected"); 
    cleanupListeners();
    resolve();
  };
  let cleanupListeners = () => {
    blockedImage.removeEventListener("load", loadListener);
    blockedImage.removeEventListener("error", errorListener);
  }

  blockedImage.addEventListener("load", loadListener);
  blockedImage.addEventListener("error", errorListener);

  document.body.appendChild(blockedImage);
});

blockedImagePromise.then(() => {
  browser.runtime.sendMessage("image-blocked");
});