getCssUnitsVal.js

const getCssUnitsVal = value => {
  const tester = document.createElement('div');
  let size;
  tester.style.width = value;
  document.body.appendChild(tester);
  size = tester.offsetWidth;
  document.body.removeChild(tester);
  return size;
};

window.addEventListener('resize', () => {
  console.log('10vw', getCssUnitsVal('10vw'));
});