getIEVersionInt

formatting for 1st answer on https://stackoverflow.com/questions/31757852/how-can-i-detect-internet-explorer-ie-and-microsoft-edge-using-javascript/32107845

const ieVersionInt = (function getIEVersionInt(){
    var rv = -1; // Return value assumes failure.

    if (navigator.appName == 'Microsoft Internet Explorer') {
       let ua = navigator.userAgent,
           re  = new RegExp("MSIE ([0-9]{1,}[\\.0-9]{0,})");

       if (re.exec(ua) !== null){
         rv = parseFloat( RegExp.$1 );
       }
    } else if(navigator.appName == "Netscape"){                       
       // in IE 11 the navigator.appVersion says 'trident'
       // in Edge the navigator.appVersion does not say trident
       if (navigator.appVersion.indexOf('Trident') === -1) rv = 12;
       else rv = 11;
    }       

    return rv;          
})();