function(window, undefined)的意义

var num = 10000
for (var i = 0; i++; i < num) {
    eval("pp" + i + "=" + i);
}

(function(window, undefined){
    var a1 = new Date();
    for (var i = 0; i < num; i++) {
        document.write(window["pp" + i]);
    }
    var a2 = new Date();
    alert(a2.getTime() - a1.getTime());
})(window);

(function(undefined){
    var a1 = new Date();
    for (var i = 0; i < num; i++) {
        document.write(window["pp" + i]);
    }
    var a2 = new Date();
    alert(a2.getTime() - a1.getTime());
})();

速度提升2/3

来自：http://blog.csdn.net/qq1119753812/article/details/6820697

PS：在一间屋子里找一个杯子，和在茶几上找一个杯子；