viewport.js

function getWindowDims() {
  let docs = document,
    win = window;
  let docsEl = (docs.compatMode && docs.compatMode === 'CSS1Compat') ?
    docs.documentElement : docs.body;
  let width = docsEl.clientWidth;
  let height = docsEl.clientHeight;
  if (win.innerWidth && width > win.innerWidth) {
    width = win.innerWidth;
    height = win.innerHeight;
  }
  return {
    width: width,
    height: height
  };
}


let win = getWindowDims();
console.log(win);