how to detect if viewport unit is supported by browser?

[hqqxxf]

`

var isBuggyIE = /MSIE [0-9]\./i.test(userAgent);
var isOldIE = /MSIE [0-8]\./i.test(userAgent);
var isOperaMini = userAgent.indexOf('Opera Mini') > -1;
var isMobileSafari = /(iPhone|iPod|iPad).+AppleWebKit/i.test(userAgent) && (function() {
  // Regexp for iOS-version tested against the following userAgent strings:
  // Example WebView UserAgents:
  // * iOS Chrome on iOS8: "Mozilla/5.0 (iPad; CPU OS 8_1 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) CriOS/39.0.2171.50 Mobile/12B410 Safari/600.1.4"
  // * iOS Facebook on iOS7: "Mozilla/5.0 (iPhone; CPU iPhone OS 7_1_1 like Mac OS X) AppleWebKit/537.51.2 (KHTML, like Gecko) Mobile/11D201 [FBAN/FBIOS;FBAV/12.1.0.24.20; FBBV/3214247; FBDV/iPhone6,1;FBMD/iPhone; FBSN/iPhone OS;FBSV/7.1.1; FBSS/2; FBCR/AT&T;FBID/phone;FBLC/en_US;FBOP/5]"
  // Example Safari UserAgents:
  // * Safari iOS8: "Mozilla/5.0 (iPhone; CPU iPhone OS 8_0 like Mac OS X) AppleWebKit/600.1.3 (KHTML, like Gecko) Version/8.0 Mobile/12A4345d Safari/600.1.4"
  // * Safari iOS7: "Mozilla/5.0 (iPhone; CPU iPhone OS 7_0 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11A4449d Safari/9537.53"
  var iOSversion = userAgent.match(/OS (\d)/);
  // viewport units work fine in mobile Safari and webView on iOS 8+
  return iOSversion && iOSversion.length > 1 && parseInt(iOSversion[1]) < 10;
})();

`

I find this code.But why not use the following method?
`

var div = document.createElement('div')
div.id = 'checkVw'
div.style.width = '50vw'
document.body.append(div)
elemWidth = parseInt(getComputedStyle(document.querySelector("#checkVw"), null).width, 10);
halfWidth = parseInt(window.innerWidth / 2, 10);
console.log("Your browser" + ((elemWidth == halfWidth) ? "" : "does not ") + " support VW and VH");

`

[hqqxxf]

@rodneyrehm