JS 客户端检测

能力检测

能力检测的目标不是识别特定的浏览器，而是识别浏览器的能力。

能力检测需要注意两点：

1. 先检测达成目的的最常用的特性。因为先检测最常用的特性可以保证代码最优化，因为在多数情况下都可以避免测试多个条件
2. 必须测试实际要用到的特性。

举例来说，检测某个属性是否存在并不能确定对象是否支持排序。看了下面的代码，得到的结论就是尽量使用typeof进行能力检测

        function isSortable(object) {
            return !!object.sort;//这样做是不取的  这不是能力检测  只检测是否存在相应的方法
        }
        //如果用上面的方法  那下面的demo不是...
        var result = isSortable({ sort: true });
        //这样更好，因为实际检测了sort是否为函数
        function isSortable(object) {
            return typeof object.sort == 'function';
        }

下面的这个方法是相应来说检测能力比较保险的方式

        /*
        *@desc:在浏览器环境下测试任何对象的某个特性是否存在
        */
        function isHostMethod(object,property) {
            var t = typeof object[property];
            return t == 'function' || (!!(t == 'object' && object[property])) || t == 'unknown';
        }

下面的这个demo是检测浏览器是否支持Netscape 风格的插件和是否具有DOM1 级规定的能力

        //need to wait until onload so body is available
        window.onload = function(){

            function getWindowWidth(){
                if (window.innerWidth){
                    return window.innerWidth;
                } else if (document.documentElement.clientWidth){
                    return document.documentElement.clientWidth;
                } else if (document.body.clientWidth){
                    return document.body.clientWidth;
                }
            }

            //determine if the browser has Netscape-style plugins
            var hasNSPlugins = !!(navigator.plugins && navigator.plugins.length);

            //determine if the browser has basic DOM Level 1 capabilities
            var hasDOM1 = !!(document.getElementById && document.createElement &&
                           document.getElementsByTagName);

            alert("Window width: " + getWindowWidth());
            alert("Supports Netscape plugins? " + hasNSPlugins);
            alert("Supports Core DOM 1? " + hasDOM1);
        }

怪癖检测

IE8及更早版本的怪癖：如果某个实例属性与[Enumerable]标识为false的某个原型属性同名，那个该实例属性将不会出现在for-in循环中

        var hasDontEnumQuirk = function(){

            var o = { toString : function(){} };
            for (var prop in o){
                if (prop == "toString"){
                    return false;
                }
            }

            return true;
        }();
        alert(hasDontEnumQuirk);

Safari3怪癖：会枚举被隐藏的属性

        var hasEnumShadowsQuirk = function(){

            var o = { toString : function(){} };
            var count = 0;
            for (var prop in o){
                if (prop == "toString"){
                    count++;
                }
            }

            return (count > 1);
        }();

        alert(hasEnumShadowsQuirk);

用户代码检测

用到的client.js文件如下：

var client = function(){

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

        //complete version
        ver: null
    };

    //browsers
    var browser = {

        //browsers
        ie: 0,
        firefox: 0,
        safari: 0,
        konq: 0,
        opera: 0,
        chrome: 0,

        //specific version
        ver: null
    };

    //platform/device/OS
    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 systems
        wii: false,
        ps: 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 == "X11") || (p.indexOf("Linux") == 0);

    //detect windows operating systems
    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"]);
        }
    }

    //determine 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;  //can't really detect - so guess
        }
    }

    //determine Android version
    if (/Android (\d+\.\d+)/.test(ua)){
        system.android = parseFloat(RegExp.$1);
    }

    //gaming systems
    system.wii = ua.indexOf("Wii") > -1;
    system.ps = /playstation/i.test(ua);

    //return it
    return {
        engine:     engine,
        browser:    browser,
        system:     system
    };

}();

浏览器检测的demo  HTML页面的js代码如下：

<!DOCTYPE html>
<html>
<head>
    <title>User-agent Detection Example</title>
    <script type="text/javascript" src="client.js"></script>
</head>
<body>
    <h2>Rendering Engines</h2>
    <ul>
        <li>client.engine.ie =
        <script>document.write(client.engine.ie);</script></li>
        <li>client.engine.webkit =
        <script>document.write(client.engine.webkit);</script></li>
        <li>client.engine.gecko =
        <script>document.write(client.engine.gecko);</script></li>
        <li>client.engine.khtml =
        <script>document.write(client.engine.khtml);</script></li>
        <li>client.engine.opera =
        <script>document.write(client.engine.opera);</script></li>
        <li>client.engine.ver =
        <script>document.write(client.engine.ver);</script></li>

    </ul>
    <h2>Browsers</h2>
    <ul>
        <li>client.browser.ie =
        <script>document.write(client.browser.ie);</script></li>
        <li>client.browser.safari =
        <script>document.write(client.browser.safari);</script></li>
        <li>client.browser.firefox =
        <script>document.write(client.browser.firefox);</script></li>
        <li>client.browser.konq =
        <script>document.write(client.browser.konq);</script></li>
        <li>client.browser.opera =
        <script>document.write(client.browser.opera);</script></li>
        <li>client.browser.chrome =
        <script>document.write(client.browser.chrome);</script></li>
        <li>client.browser.ver =
        <script>document.write(client.browser.ver);</script></li>
    </ul>

    <h2>System</h2>
    <ul>
        <li>client.system.win =
        <script>document.write(client.system.win);</script></li>
        <li>client.system.mac =
        <script>document.write(client.system.mac);</script></li>
        <li>client.system.x11 =
        <script>document.write(client.system.x11);</script></li>
        <li>client.system.iphone =
        <script>document.write(client.system.iphone);</script></li>
        <li>client.system.ipod =
        <script>document.write(client.system.ipod);</script></li>
        <li>client.system.ipad =
        <script>document.write(client.system.ipad);</script></li>
        <li>client.system.ios =
        <script>document.write(client.system.ios);</script></li>
        <li>client.system.android =
        <script>document.write(client.system.android);</script></li>
        <li>client.system.nokiaN =
        <script>document.write(client.system.nokiaN);</script></li>
        <li>client.system.winMobile =
        <script>document.write(client.system.winMobile);</script></li>
        <li>client.system.wii =
        <script>document.write(client.system.wii);</script></li>
        <li>client.system.ps =
        <script>document.write(client.system.ps);</script></li>
    </ul>
    <center>

    </center>
</body>
</html>

总结一下：