网页屏蔽Backspace事件，输入框不屏蔽

document.onkeydown = function (e) {
     var code;
     if (!e){ var e = window.event;}
     if (e.keyCode){ code = e.keyCode;}
     else if (e.which){ code = e.which;}
     //BackSpace 8;
     if (
       (event.keyCode == 8)
       && ((event.srcElement.type != "text" && event.srcElement.type != "textarea" && event.srcElement.type != "password")
         ||  event.srcElement.readOnly == true
         )

      ) {

      event.keyCode = 0;
      event.returnValue = false;
     }
     return true;
};