JSFiddle - React, Tailwind, and code Playground

JavaScript

function getViewportSize () {
  var w, h, el;
  if (window.innerWidth) {
    w = window.innerWidth;
    h = window.innerHeight;
  } else if ((el = document.documentElement) && (el.clientWidth || el.clientHeight)) {
    w = el.clientWidth;
    h = el.clientHeight;
  } else if((el = document.body) && (el.clientWidth || el.clientHeight)) {
    w = el.clientWidth;
    h = el.clientHeight;
  }
  return { width: w, height: h };
}

var viewport = getViewportSize();
if (viewport.width < 900 || viewport.height < 820) {
   alert('klein! ' + viewport.width + 'x' + viewport.height);
}