js禁用Backspace退格键代码实例下载

<script>
function banBackSpace(e) {  
  var ev = e || window.event;
  var obj = ev.target || ev.srcElement;
  var t = obj.type || obj.getAttribute('type');
  //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;
</script>