getStyleFromProperty.js

const getStyle = (classOrId, property) => {
  var firstChar = classOrId.charAt(0);
  var remaining= classOrId.substring(1);
  var elem = (firstChar =='#') ?  document.getElementById(remaining) : document.getElementsByClassName(remaining)[0];

  if(elem) {
    return window.getComputedStyle(elem).getPropertyValue(property);
  }
  return;
}

//usage
let containerWidth= getStyle('.container', 'width');