利用闭包重写document.createElement函数

document.createElement = (function(fn){
    return function(type,id,className){
    var elem = fn.call(document,type);
    if(id) elem.id = id;
    if(className) elem.className = className;
    return elem;
}
})(document.createElement);

var t = document.createElement("div","abc","ddd");