验证浏览器是什么东东的代码,来至《JavaScript高级程序设计》一书
var sUserAgent = navigator.userAgent;
var fAppVersion = parseFloat(navigator.appVersion);


function compareVersion(sVersion1, sVersion2) ...{

var aVersion1 = sVersion.split(".");
var aVersion2 = sVersion.split(".");


if (aVersion1.length > aVersion2.length) ...{

for (var i=0; i < aVersion1.length - aVersion2.length; i++) ...{
aVersion2.push("0");
}

} else if (aVersion1.length < aVersion2.length) ...{

for (var i=0; i < aVersion2.length - aVersion1.length; i++) ...{
aVersion1.push("0");
}
}


for (var i=0; i < aVersion1.length; i++) ...{


if (aVersion1[i] < aVersion2[i]) ...{
return -1;

} else if (aVersion1[i] > aVersion2[i]) ...{
return 1;
}
}

return 0;
}

var isOpera = sUserAgent.indexOf("Opera") > -1;
var isMinOpera4 = isMinOpera5 = isMinOpera6 = isMinOpera7 = isMinOpera7_5 = false;


if (isOpera) ...{
var fOperaVersion;

if (navigator.appName == "Opera") ...{
fOperVersion = fAppVersion;

} else ...{
var reOperaVersion = new RegExp("Opera (/d+/./d+)");
reOperaVersion.test(sUserAgent);
fOperationVersion = parseFlot(RegExp["$1"]);
}

isMinOpera4 = fOperaVersion >= 4;
isMinOpera5 = fOperaVersion >= 5;
isMinOpera6 = fOperaVersion >= 6;
isMinOpera7 = fOperaVersion >= 7;
isMinOpera7_5 = fOperaVersion >= 7.5;
}


var isKHTML = sUserAgent.indexOf("KHTML") > -1 || sUserAgent.indexOf("Konqueror") > -1 || sUserAgent.indexOf("AppleWebKit") > -1;
var isMinSafari1 = isMinSafari1_2 = false;
var isMinKonq2_2 = isMinKonq3 = isMinKonq3_1 = isMinKonq3_2 = false;


if (isKHTML) ...{
isSafari = sUserAgent.indexOf("AppleWebKit") > -1;
isKonq = sUserAgent.indexOf("Konqueror") > -1;


if (isSafari) ...{
var reAppleWebKit = new RegExp("AppleWebKit//(/d+(?:/./d*)?)");
reAppleWebKit.test(sUserAgent);
var fAppleWebKitVersion = parseFloat(RegExp["$1"]);

isMinSafari1 = fAppleWebKitVersion >= 85;
isMinSafari1_2 = fAppleWebKitVersion >= 124;

} else if (isKonq) ...{
var reKonq = new RegExp("Konqueror//(/d+(?:/./d+(?:/./d)?)?)");
reKonq.test(sUserAgent);
isMinKonq2_2 = compareVersion(RegExp["$1"], "2.2") >= 0;
isMinKonq3 = compareVersion(RegExp["$1"], "3.0") >= 0;
isMinKonq3_1 = compareVersion(RegExp["$1"], "3.1") >= 0;
isMinKonq3_2 = compareVersion(RegExp["$1"], "3.2") >= 0;
}
}

var isIE = sUserAgent.indexOf("compatible") > -1 && sUserAgent.indexOf("MSIE") > -1 && !isOpera;

var isMinIE4 = isMinIE5 = isMinIE5_5 = isMinIE6 = false;


if (isIE) ...{
var reIE = new RegExp("MSIE (/d+/./d+);");
reIE.test(sUserAgent);
var fIEVersion = parseFloat(RegExp["$1"]);

isMinIE4 = fIEVersion >= 4;
isMinIE5 = fIEVersion >= 5;
isMinIE5_5 = fIEVersion >= 5.5;
isMinIE6 = fIEVersion >= 6.0;

}

var isMoz = sUserAgent.indexOf("Gecko") > -1;
var isMinMoz1 = isMinMoz1_4 = isMinMoz1_5 = false;


if (isMoz) ...{
var reMoz = new RegExp("rv:(/d+/./d+(?:/./d+)?)");
reMoz.test(sUserAgent);
isMinMoz1 = compareVersion(RegExp["$1"], "1.0") >= 0;
isMinMoz1_4 = compareVersion(RegExp["$1"], "1.4") >= 0;
isMinMoz1_5 = compareVersion(RegExp["$1"], "1.5") >= 0;
}

var isNS4 = !isIE && !isOpera && !isMoz && !isKHTML && (sUserAgent.indexOf("Mozilla") == 0) && (navigator.appName == "Netscape") && (fAppVersion >= 4.0 && fAppVersion < 5.0);
var isMinNS4 = isMinNS4_5 = isMinNS4_7 = isMinNS4_8 = false;


if (isNS4) ...{
isMinNS4 = true;
isMinNS4_5 = fAppVersion >= 4.5;
isMinNS4_7 = fAppVersion >= 4.7;
isMinNS4_8 = fAppVersion >= 4.8;
}

var isWin = (navigator.platform == "Win32") || (navigator.platform == "Windows");

var isMac = (navigator.platform == "Mac68K") || (navigator.platform == "MacPPC") || (navigator.platform == "Macintosh");

var isUnix = (navigator.platform =="X11") && !isWin && !isMac;

var isWin95 = isWin98 = isWinNT4 = isWin2K = isWinME = isWinXP = false;
var isMac68K = isMacPPC = false;
var isSunOS = isMinSunOS4 = isMinSunOS5 = isMinSunOS5_5 = false;


if (isWin) ...{
isWin95 = sUserAgent.indexOf("Win95") > -1 || sUserAgent.indexOf("Windows 95") > -1;
isWin98 = sUserAgent.indexOf("Win98") > -1 || sUserAgent.indexOf("Windows 98") > -1;
isWinME = sUserAgent.indexOf("Win 9x 4.90") > -1 || sUserAgent.indexOf("Windows ME") > -1;
isWin2K = sUserAgent.indexOf("Windows NT 5.0") > -1 || sUserAgent.indexOf("Windows 2000") > -1;
isWinXP = sUserAgent.indexOf("Windows NT 5.1") > -1 || sUserAgent.indexOf("Windows XP") > -1;
isWinNT4 = sUserAgent.indexOf("WinNT") > -1 || sUserAgent.indexOf("Windows NT") > -1 || sUserAgent.indexOf("WinNT4.0") > -1 || sUserAgent.indexOf("Windows NT 4.0") > -1 && (!isWinME && !isWin2K && !isWinXP);
}


if (isMac) ...{
isMac68K = sUserAgent.indexOf("Mac_68000") > -1 || sUserAgent.indexOf("68K") > -1;
ifMacPPC = sUserAgent.indexOf("Mac_PowerPC") > -1 || sUserAgent.indexOf("PPC") > -1;
}


if (isUnix) ...{
isSunOS = sUserAgent.indexOf("SunOS") > -1;


if (isSunOS) ...{
var reSunOS = new RegExp("SunOS (/d+/./d+(?:/./d+)?)");

reSunOS.test(sUserAgent);
isMinSunOS4 = compareVersion(RegExp["$1"], "4.0") >= 0;
isMinSunOS5 = compareVersion(RegExp["$1"], "5.0") >= 0;
isMinSunOS5_5 = compareVersion(RegExp["$1"], "5.5") >= 0;
}
}