一个比较完整的客户端判断

var client = function() {
	var engine = {
		// rendering engines
		ie : 0,
		gecko : 0,
		webkit : 0,
		khtml : 0,
		opera : 0,
		// specific version
		ver : null
	};
	var browser = {
		// browsers
		ie : 0,
		firefox : 0,
		safari : 0,
		konq : 0,
		opera : 0,
		chrome : 0,
		safari : 0,
		// specific version
		ver : null
		// detection of rendering engines/platforms/devices here
	};
	var system = {
		win : false,
		mac : false,
		x11 : false
	};
	// detect rendering engines/browsers
	var ua = navigator.userAgent;
	if (window.opera) {
		engine.ver = browser.ver = window.opera.version();
		engine.opera = browser.opera = parseFloat(engine.ver);
	} else if (/AppleWebKit\/(\S+)/.test(ua)) {
		engine.ver = RegExp['$1'];
		engine.webkit = parseFloat(engine.ver);
		// figure out if it’s Chrome or Safari
		if (/Chrome\/(\S+)/.test(ua)) {
			browser.ver = RegExp['$1'];
			browser.chrome = parseFloat(browser.ver);
		} else if (/Version\/(\S+)/.test(ua)) {
			browser.ver = RegExp['$1'];
			browser.safari = parseFloat(browser.ver);
		} else {
			// approximate version
			var safariVersion = 1;
			if (engine.webkit < 100) {
				safariVersion = 1;
			} else if (engine.webkit < 312) {
				safariVersion = 1.2;
			} else if (engine.webkit < 412) {
				safariVersion = 1.3;
			} else {
				safariVersion = 2;
			}
			browser.safari = browser.ver = safariVersion;
		}
	} else if (/KHTML\/(\S+)/.test(ua) || /Konqueror\/([^;]+)/.test(ua)) {
		engine.ver = browser.ver = RegExp['$1'];
		engine.khtml = browser.konq = parseFloat(engine.ver);
	} else if (/rv:([^\)]+)\) Gecko\/\d{8}/.test(ua)) {
		engine.ver = RegExp['$1'];
		engine.gecko = parseFloat(engine.ver);
		// determine if it’s Firefox
		if (/Firefox\/(\S+)/.test(ua)) {
			browser.ver = RegExp['$1'];
			browser.firefox = parseFloat(browser.ver);
		}
	} else if (/MSIE ([^;]+)/.test(ua)) {
		engine.ver = browser.ver = RegExp['$1'];
		engine.ie = browser.ie = parseFloat(engine.ver);
	}
	//detect browsers
	browser.ie = engine.ie;
	browser.opera = engine.opera;
    
    //detect platform
	var p = navigator.platform;
	system.win = p.indexOf('Win') == 0;
	system.mac = p.indexOf('Mac') == 0;
	system.x11 = (p.indexOf('X11') == 0) || (p.indexOf('Linux') == 0);
	if (system.win) {
		if (/Win(?:dows )?([^do]{2})\s?(\d+\.\d+)?/.test(ua)) {
			if (RegExp['$1'] == 'NT') {
				switch (RegExp['$2']) {
					case '5.0' :
						system.win = '2000';
						break;
					case '5.1' :
						system.win = 'XP';
						break;
					case '6.0' :
						system.win = 'Vista';
						break;
					default :
						system.win = 'NT';
						break;
				}
			} else if (RegExp['$1'] == '9x') {
				system.win = 'ME';
			} else {
				system.win = RegExp['$1'];
			}
		}
	}
	return {
		engine : engine,
		system : system,
		browser : browser
	};
}();

//usage:
if (client.engine.webkit) { // if it’s WebKit
	if (client.browser.chrome) {
		// do something for Chrome
	} else if (client.browser.safari) {
		// do something for Safari
	}
} else if (client.engine.gecko) {
	if (client.browser.firefox) {
		// do something for Firefox
	} else {
		// do something for other Gecko browsers
	}
}

整理自<<Professional.JavaScript.for.Web.Developers.2nd.Edition>>