In page notification overlay for chrome extension

pageNotification.js

// In page notification overlay for chrome extension.
// Please use page or browser action instead if possible.
// add pageNotification.js to web_accessible_resources key in manifest.json to allow iframe page to load it
// uses postMessage so might mess with overlayed page

const pageNotification = {
  // call in background script etc to show overlay notification page in tab
  show: (tabId, url) => {
    chrome.tabs.executeScript(tabId, {file: 'pageNotification.js'}, function() {
      chrome.tabs.executeScript(tabId, {code: 'pageNotification.inject(' + JSON.stringify(url) + ');'})
    });
  },

  // call in notification (iframe) page to close
  close: () => {
    // TODO: hack to communicate close (will trigger load event in parent iframe)
    window.location.href = "";
  },

  // call in notification (iframe) page to tell parent to resize iframe
  postResizeOnload: () => {
    window.addEventListener('load', function(e) {
      let s = document.defaultView.getComputedStyle(document.body);

      // TODO: not perfect measure but seems to work in most cases
      let width = (
        parseInt(s.getPropertyValue('margin-left')) +
        document.documentElement.scrollWidth +
        parseInt(s.getPropertyValue('margin-right'))
      );
      let height = document.documentElement.scrollHeight;

      window.parent.postMessage({width: width, height: height}, '*');
    });
  },

  // used by pageNotification.show
  inject: (url) => {
    const elmId = '___pageNotification_' + chrome.runtime.id;

    // recreate if exist
    let elm = document.getElementById(elmId);
    if (elm) {
      document.body.removeChild(elm);
    }

    elm = document.createElement('iframe');
    elm.id = elmId;
    elm.src = chrome.extension.getURL(url);
    elm.frameBorder = 0;
    elm.style.position = 'fixed';
    elm.style['z-index'] = 2147483647;
    elm.style.right = '10px';
    elm.style.top = '10px';

    let elmResizeFn = function(message) {
      if (chrome.extension.getURL('').indexOf(message.origin) !== 0) {
        return;
      }

      elm.style.width = message.data.width + 'px';
      elm.style.height = message.data.height + 'px';
      window.removeEventListener('message', elmResizeFn);

      // TODO: hack to communicate close
      elm.addEventListener('load', (e) => {
        document.body.removeChild(elm);
      });
    };
    window.addEventListener('message', elmResizeFn);

    document.body.appendChild(elm);
  }
}