请达人帮忙解释下下面这段用来识别浏览器的代码，如何使用？

<script>

/**********************************************************************

    * Helper function to parse the user agent.  Sets the following form xiaonan 2014-07-11 16:59:19

    * .os.weixin

    * .os.webkit

    * .os.android

    * .os.ipad

    * .os.iphone

    * .os.webos

    * .os.touchpad

    * .os.blackberry

    * .os.opera

    * .os.fennec

    * .os.ie

    * .os.ieTouch

    * .os.supportsTouch

    * .os.playbook

    * .os.tizen

    * .feat.nativeTouchScroll

    * @api private

    if($.os.webkit){

        alert("I'm webkit browser");

    };

**********************************************************************/

function detectUA($, userAgent) {

    $.os = {};

    $.os.weixin = userAgent.match(/MicroMessenger\/([\d.]+)/) ? true : false;

    $.os.webkit = userAgent.match(/WebKit\/([\d.]+)/) ? true : false;

    $.os.android = userAgent.match(/(Android)\s+([\d.]+)/) || userAgent.match(/Silk-Accelerated/) ? true : false;

    $.os.androidICS = $.os.android && userAgent.match(/(Android)\s4/) ? true : false;

    $.os.ipad = userAgent.match(/(iPad).*OS\s([\d_]+)/) ? true : false;

    $.os.iphone = !$.os.ipad && userAgent.match(/(iPhone\sOS)\s([\d_]+)/) ? true : false;

    $.os.ios7 = ($.os.ipad||$.os.iphone)&&userAgent.match(/7_/) ? true : false;

    $.os.webos = userAgent.match(/(webOS|hpwOS)[\s\/]([\d.]+)/) ? true : false;

    $.os.touchpad = $.os.webos && userAgent.match(/TouchPad/) ? true : false;

    $.os.ios = $.os.ipad || $.os.iphone;

    $.os.playbook = userAgent.match(/PlayBook/) ? true : false;

    $.os.blackberry10 = userAgent.match(/BB10/) ? true : false;

    $.os.blackberry = $.os.playbook || $.os.blackberry10|| userAgent.match(/BlackBerry/) ? true : false;

    $.os.chrome = userAgent.match(/Chrome/) ? true : false;

    $.os.opera = userAgent.match(/Opera/) ? true : false;

    $.os.fennec = userAgent.match(/fennec/i) ? true : userAgent.match(/Firefox/) ? true : false;

    $.os.ie = userAgent.match(/MSIE 10.0/i)||userAgent.match(/Trident\/7/i) ? true : false;

    $.os.ieTouch = $.os.ie && userAgent.toLowerCase().match(/touch/i) ? true : false;

    $.os.tizen = userAgent.match(/Tizen/i)?true:false;

    $.os.supportsTouch = ((window.DocumentTouch && document instanceof window.DocumentTouch) || "ontouchstart" in window);

    $.os.kindle=userAgent.match(/Silk-Accelerated/)?true:false;

    //features

    $.feat = {};

    var head = document.documentElement.getElementsByTagName("head")[0];

    $.feat.nativeTouchScroll = typeof(head.style["-webkit-overflow-scrolling"]) !== "undefined" && ($.os.ios||$.os.blackberry10);

    $.feat.cssPrefix = $.os.webkit ? "Webkit" : $.os.fennec ? "Moz" : $.os.ie ? "ms" : $.os.opera ? "O" : "";

    $.feat.cssTransformStart = !$.os.opera ? "3d(" : "(";

    $.feat.cssTransformEnd = !$.os.opera ? ",0)" : ")";

    if ($.os.android && !$.os.webkit)

    $.os.android = false;

    var items=["Webkit","Moz","ms","O"];

    for(var j=0;j<items.length;j++){

        if(document.documentElement.style[items[j]+"Transform"]==="")

        $.feat.cssPrefix=items[j];

    }

}

detectUA($, navigator.userAgent);

$.__detectUA = detectUA; 

</script>