function banBackSpace(e) {                  var ev = e || window.event;//获取event对象                  var obj = ev.target || ev.srcElement;//获取事件源                  var t = obj.type || obj.getAttribute('type');//获取事件源类型                  //获取作为判断条件的事件类型                  var vReadOnly = obj.readOnly;                  var vDisabled = obj.disabled;                  //处理undefined值情况                  vReadOnly = (vReadOnly == undefined) ? false : vReadOnly;                  vDisabled = (vDisabled == undefined) ? true : vDisabled;                  //当敲Backspace键时，事件源类型为密码或单行、多行文本的，                  //并且readOnly属性为true或disabled属性为true的，则退格键失效                  var flag1 = ev.keyCode == 8 && (t == "password" || t == "text" || t == "textarea") && (vReadOnly == true || vDisabled == true);                  //当敲Backspace键时，事件源类型非密码或单行、多行文本的，则退格键失效                  var flag2 = ev.keyCode == 8 && t != "password" && t != "text" && t != "textarea";                  //判断                  if (flag2 || flag1) return false              }              //禁止退格键 作用于Firefox、Opera              document.onkeypress = banBackSpace;              //禁止退格键 作用于IE、Chrome              document.onkeydown = banBackSpace;