更可靠的能力检测

在浏览器环境下测试任何队形的某个特性是否存在，要使用下面这个函数：

// 在IE中会导致错误
var xhr = new ActiveXObject("Microsoft.XMLHttp");
// 这里会发生错误
if(xhr.open) {
  // 执行操作
}

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

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

用户代理检测

var client = function () {
    var engine = {
        ie: 0,
        gecko: 0,
        webkit: 0,
        khtml: 0,
        opera: 0,
        ver: null
    };

    var browser = {
        ie: 0,
        firefox: 0,
        safari: 0,
        konq: 0,
        opera: 0,
        chrome: 0,
        ver: null
    };

    var system = {
        win: false,
        mac: false,
        xll: false,
        iphone: false,
        ipoad: false,
        ipad: false,
        ios: false,
        android: false,
        nokiaN: false,
        winMobile: false,
        wii: false,
        ps: false
    };

    var ua = navigator.userAgent;
    // 检测浏览器呈现引擎
    if (window.opera) {
        engine.ver = browser.ver = window.opera.version();
        engine.opera = browser.opera = parseFloat(engine.ver);
    } else if (/AppleWebkit\/(\S+)/i.test(ua)) {
        engine.ver = RegExp['$1'];
        engine.webkit = parseFloat(engine.ver);

        // 确定是Chrome还是Safari
        if (/Chrome\/(\S+)/i.test(ua)) {
            browser.ver = RegExp['$1'];
            browser.chrome = parseFloat(browser.ver);
        } else if (/Version\/(\S+)/i.test(ua)) {
            browser.ver = RegExp['$1'];
            browser.safari = parseFloat(browser.ver);
        } else {
            // 近似地确定版本号
            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.safari = safariVersion;
        }
    } else if (/KHTML\/(\S+)/i.test(ua) || /Konqueror\/([^;]+)/i.test(ua)) {
        engine.ver = browser.ver = RegExp['$1'];
        engine.khtml = browser.konq = parseFloat(engine.ver);
    } else if (/rv:([^\)]+)\) Gecko\/\d{8}/i.test(ua)) {
        engine.ver = RegExp['$1'];
        engine.gecko = parseFloat(engine.ver);

        // 确定是不是Firefox
        if (/Firefox\/(\S+)/i.test(ua)) {
            engine.ver = browser.ver = RegExp['$1'];
            engine.firefox = parseFloat(browser.ver);
        }
    } else if (/MSIE ([^;]+)/i.test(ua)) {
        engine.ver = browser.ver = RegExp['$1'];
        engine.ie = browser.ie = parseFloat(engine.ver);
    }

    // 检测平台
    var p = navigator.platform;
    system.win = p.indexOf('Win') == 0;
    system.mac = p.indexOf('Mac') == 0;
    system.xll = (p.indexOf('Xll') == 0 || p.indexOf('Linux') == 0);

    // 检测Windows操作系统
    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;
                    case '6.2':
                        system.win = '8';
                        break;
                    default:
                        system.win = 'NT';
                        break;
                }
            } else if (RegExp['$1'] == '9x') {
                system.win = 'ME';
            } else {
                system.win = RegExp['$1'];
            }
        }
    }

    // 移动设备
    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)/i.test(ua)) {
            system.win = 'Phone';
            system.winMobile = parseFloat(RegExp['$1']);
        }
    }

    // 检测iOS版本
    if (system.mac && ua.indexOf('Mobile') > -1) {
        if (/CPU (?:iPhone )?OS (\d+_\d+)/i.test(ua)) {
            system.ios = parseFloat(RegExp['$1'].replace('_', '.'));
        } else {
            system.ios = 2; // 不能真正检测出来，所以只能猜测
        }
    }

    // 检测Android版本
    if (/Android (\d+\.\d+)/i.test(ua)) {
        system.android = parseFloat(RegExp['$1']);
    }

    // 游戏系统
    system.wii = ua.indexOf('Wii') > -1;
    system.ps = /PlayStation/i.test(ua);

    return {
        engine: engine,
        browser: browser,
        system: system
    };
}();