JS禁止后退键(Backspace)

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//处理键盘事件 禁止后退键(Backspace)密码或单行、多行文本框除外       
function forbidBackSpace(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;      
  }              
//禁止后退键  作用于IE、Chrome       
document.onkeydown = forbidBackSpace;