EXT 中js语法请教

ext.js文件中有这样一段js代码

(function(){
    var ua = navigator.userAgent.toLowerCase();

    var isStrict = document.compatMode == "CSS1Compat",
        isOpera = ua.indexOf("opera") > -1,
        isSafari = (/webkit|khtml/).test(ua),
        isIE = ua.indexOf("msie") > -1,
        isIE7 = ua.indexOf("msie 7") > -1,
        isGecko = !isSafari && ua.indexOf("gecko") > -1,
        isBorderBox = isIE && !isStrict,
        isWindows = (ua.indexOf("windows") != -1 || ua.indexOf("win32") != -1),
        isMac = (ua.indexOf("macintosh") != -1 || ua.indexOf("mac os x") != -1),
        isLinux = (ua.indexOf("linux") != -1),
        isSecure = window.location.href.toLowerCase().indexOf("https") === 0;
})();

这段代码应该是加载后自动执行，不知道这没写的用以何在？是不是为了防止命名冲突？