跨iframe获取文档对象

函数体如下：

function subFrameDoc(iframeId){
 try{
  var frameDoc;

  if(document.all){
      try{
   frameDoc = window.frames[iframeId].document;
         }catch(e){
          try{
    frameDoc = parent.window.frames[iframeId].document;
          }catch(e){}//alert("Your IE´s version is to old!\nPlease try some new one!");
      }
  }else{
      try{
   frameDoc = document.getElementById(iframeId).contentWindow.document;
      }catch(e){
   try{
    frameDoc = parent.document.getElementById(iframeId).contentWindow.document;
   }catch(e){}//alert("Your Firefox´s version is to old!\nPlease try some new one!");
      }
  }

  return frameDoc ? frameDoc : null;
 }catch(e){return false;}
}