Get real width and height using getComputedStyle

getStyle.js

  // ref. https://developer.mozilla.org/en-US/docs/Web/API/Window/getComputedStyle
  function getStyle(elem, prop) {
    var styles = window.getComputedStyle(elem, null);
    var value = styles.getPropertyValue(prop);
    if (!value) {
      throw "Could not get style for " + prop;
    }
    return value
  }