Wie erkennt Javascript, ob es sich um einen IE-Browser handelt?

function IEVersion() {
      var userAgent = navigator.userAgent; //取得浏览器的userAgent字符串
      var isIE = userAgent.indexOf("compatible") > -1 && userAgent.indexOf("MSIE") > -1; //判断是否IE<11浏览器  
      var isEdge = userAgent.indexOf("Edge") > -1 && !isIE; //判断是否IE的Edge浏览器  
      var isIE11 = userAgent.indexOf(&#39;Trident&#39;) > -1 && userAgent.indexOf("rv:11.0") > -1;
      if(isIE) {
          var reIE = new RegExp("MSIE (\\d+\\.\\d+);");
          reIE.test(userAgent);
          var fIEVersion = parseFloat(RegExp["$1"]);
          if(fIEVersion == 7) {
              return 7;
          } else if(fIEVersion == 8) {
              return 8;
          } else if(fIEVersion == 9) {
              return 9;
          } else if(fIEVersion == 10) {
              return 10;
          } else {
              return 6;//IE版本<=7
          }   
      } else if(isEdge) {
          return &#39;edge&#39;;//edge
      } else if(isIE11) {
          return 11; //IE11  
      }else{
          return -1;//不是ie浏览器
      }
  }

    function isIE() {
        if(!!window.ActiveXObject || "ActiveXObject" in window){
          document.body.innerHTML = &#39;<h1>你的浏览器已过时，请更新你的浏览器</h1>&#39;;
          return true;
        }else{
          return false;
    　　 }
    }