function StringBuffer(str){    var arr = [];    str = str || "";    arr.push(str);    this.append = function(str1)    {        arr.push(str1);        return this;    };    this.toString = function()    {        return arr.join("");    };};document.onreadystatechange = function(){    if(document.readyState == "complete")        {        var sb = new StringBuffer();        sb.append("Hello,").append("World");        alert(sb.toString());    }};