得到用户控件byID

for(i=2;i<=6;i++)
{
      if(document.getElementById("dg__ctl"+String(i)+"_JSDJ").value != "")
      {
           document.getElementById("dg__ctl"+String(i)+"_SBSL").value = "0.000";
           document.getElementById("dg__ctl"+String(i)+"_JSJE").value = "0.000";
      }
}