function stringbuilder()
{
  this.arr = new Array();
  this.append=function(str)
  {
    this.arr.push(str);
  }
  this.tostring = function()
  {
    return this.arr.join('');
  }
}


var sb = new stringbuilder();
sb.append("hello");
sb.append("doudou");
$("#div1").html(sb.tostring());