function StringBuffer() { 
  this._strs = new Array; 
} 
StringBuffer.prototype.append = function (str) { 
  this._strs.push(str); 
}; 
StringBuffer.prototype.toString = function() { 
  return this._strs.join(""); 
};

var sb=new StringBuffer();

sb.append("sss")

sb.append("ddd");

sb.toString(); //"sssddd"

/**
*  格式化字符串
*  format("{0},{1}","ddd","fff");
*  format('<button id="{0}" type="{1}">{2}</button>',"btnOk","Button","Ok")
**/
function format(str){
  for(var i=0;i<arguments.length-1;i++){
    var placeHolder="{"+i+"}";
    if(str.indexOf(placeHolder)!=-1){
      str=str.replace(placeHolder,arguments[i+1]);
    }
  }
  return str;
}