JS实现点击下载

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

 <a   href="a.txt"   id=pic1   onclick="savepic();return false;"   style="cursor:hand">点击下载</a>  
          <script>  
                  function   savepic()
                {  
                  if(document.all.a1==null)
                        {  
                                  objIframe=document.createElement("IFRAME");  
                                  document.body.insertBefore(objIframe);  
                                  objIframe.outerHTML=   "<iframe   name=a1   style='width:0;hieght:0'   src="+pic1.href+"></iframe>";  
                                  re=setTimeout("savepic()",1)  
                          }  
                 else
                        {  
                                  clearTimeout(re)  
                                  pic   =   window.open(pic1.href,"a1")  
                                  pic.document.execCommand("SaveAs")  
                                  document.all.a1.removeNode(true)  
                          }
                }  
          </script>