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){}