JS判断IE6,IE7,IE8,FF,Safari

JQuery中判断浏览器使用$.browser.msie,Extjs中判断使用Ext.isIE?

var isIE = !!window.ActiveXObject;
var isIE6 = isIE && !window.XMLHttpRequest;
var isIE8 = isIE && !!document.documentMode;
var isIE7 = isIE && !isIE6 && !isIE8;

平台检测代码Ext源码：

var Browser = {};
try{
    (function(){
        var idSeed = 0,
        ua = navigator.userAgent.toLowerCase(),
        check = function(r){
            return r.test(ua);
        },
        DOC = document,
        isStrict = DOC.compatMode == "CSS1Compat",
        isOpera = check(/opera/),
        isChrome = check(/\bchrome\b/),
        isWebKit = check(/webkit/),
        isSafari = !isChrome && check(/safari/),
        isSafari2 = isSafari && check(/applewebkit\/4/), // unique to Safari 2
        isSafari3 = isSafari && check(/version\/3/),
        isSafari4 = isSafari && check(/version\/4/),
        isIE = !isOpera && check(/msie/),
        isIE7 = isIE && check(/msie 7/),
        isIE8 = isIE && check(/msie 8/),
        isIE6 = isIE && !isIE7 && !isIE8,
        isGecko = !isWebKit && check(/gecko/),
        isGecko2 = isGecko && check(/rv:1\.8/),
        isGecko3 = isGecko && check(/rv:1\.9/),
        isBorderBox = isIE && !isStrict,
        isWindows = check(/windows|win32/),
        isMac = check(/macintosh|mac os x/),
        isAir = check(/adobeair/),
        isLinux = check(/linux/),
        isIpad = check(/ipad/),
        isSecure = /^https/i.test(window.location.protocol);
        extend(Browser,{
            isOpera:isOpera,
            isIE:isIE,
            isIE6:isIE6,
            isFirefox:isGecko,
            isSafari:isSafari,
            isChrome:isChrome,
            isIpad:isIpad
        });
    })();
}catch(e){}

但是很多情况下需要区分不同浏览器，所以需要js方法判断。