Extract a websites custom global variables. Very not-tested.

getSiteVars.js

const getSiteVars = () => {
  const blankWindow = window.open('about:blank');
  const defaultKeys = Object.keys(blankWindow);
  blankWindow.close();
  return Object
    .keys(window)
    .reduce((all, key) => 
      blankWindow.hasOwnProperty(key) ? all : { ...all, [key]: window[key]}
      , {}
    );
};