1. Capability test

The goal of capability testing is not to identify a specific browser, but to identify the capability of the browser. When possible, try to use typeof for capability testing. To test whether a property of any object exists in the browser environment, use the following function:

function isHostMethod(object,property) {
    var t = typeof object[property];
    return t=='function' || (!!(t=='object' && object[[property]])) || t=='unknown';
}

result = isHostMethod(xhr,"open");   //true
result = isHostMethod(xhr,"foo");    //false

2. Quirky test

The goal of quirks detection is to identify the particularity of the browser. Quirks are actually bug s in the browser implementation.

3. User agent detection

Identify the browser by detecting the user agent string. The user agent string contains a lot of browser related information, including browser, platform, operating system and browser version.
(1) recognition and presentation engine
(2) identify browser
(3) identification platform
(4) identify the Winidows operating system
(5) identify mobile devices
(6) identify game system

The complete code is as follows:

var client = function(){

    //Rendering engine
    var engine = {            
        ie: 0,
        gecko: 0,
        webkit: 0,
        khtml: 0,
        opera: 0,

        //Full version number
        ver: null  
    };
    
    //Browser
    var browser = {
        
        //Main browser
        ie: 0,
        firefox: 0,
        safari: 0,
        konq: 0,
        opera: 0,
        chrome: 0,

        //Specific version No.
        ver: null
    };

    
    //Platforms, devices and operating systems
    var system = {
        win: false,
        mac: false,
        x11: false,
        
        //Mobile devices
        iphone: false,
        ipod: false,
        ipad: false,
        ios: false,
        android: false,
        nokiaN: false,
        winMobile: false,
        
        //Game system
        wii: false,
        ps: false 
    };    

    //Detect rendering engine and browser
    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);
        
        //Determine whether 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 {
            //Approximately determine version good = number
            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);
        
        //Make sure 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 browser
    browser.ie = engine.ie;
    browser.opera = engine.opera;
    

    //Detection platform
    var p = navigator.platform;
    system.win = p.indexOf("Win") == 0;
    system.mac = p.indexOf("Mac") == 0;
    system.x11 = (p == "X11") || (p.indexOf("Linux") == 0);

    //Testing windows operating system
    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;
                    case "6.1":
                        system.win = "7";
                        break;
                    default:
                        system.win = "NT";
                        break;                
                }                            
            } else if (RegExp["$1"] == "9x"){
                system.win = "ME";
            } else {
                system.win = RegExp["$1"];
            }
        }
    }
    
    //Mobile devices
    system.iphone = ua.indexOf("iPhone") > -1;
    system.ipod = ua.indexOf("iPod") > -1;
    system.ipad = ua.indexOf("iPad") > -1;
    system.nokiaN = ua.indexOf("NokiaN") > -1;
    
    //windows mobile
    if (system.win == "CE"){
        system.winMobile = system.win;
    } else if (system.win == "Ph"){
        if(/Windows Phone OS (\d+.\d+)/.test(ua)){;
            system.win = "Phone";
            system.winMobile = parseFloat(RegExp["$1"]);
        }
    }
    
    
    //Detect iOS version
    if (system.mac && ua.indexOf("Mobile") > -1){
        if (/CPU (?:iPhone )?OS (\d+_\d+)/.test(ua)){
            system.ios = parseFloat(RegExp.$1.replace("_", "."));
        } else {
            system.ios = 2;  //We can't really detect it, so we can only guess.
        }
    }
    
    //Check Android version
    if (/Android (\d+\.\d+)/.test(ua)){
        system.android = parseFloat(RegExp.$1);
    }
    
    //Game system
    system.wii = ua.indexOf("Wii") > -1;
    system.ps = /playstation/i.test(ua);
    
    //Return these objects
    return {
        engine:     engine,
        browser:    browser,
        system:     system        
    };

}();