jquery阻止用户点Backspace返回上一页

$(document).keydown(function(e){
   var target = e.target ;
   var tag = e.target.tagName.toUpperCase();
   if(e.keyCode == 8){
    if((tag == 'INPUT' && !$(target).attr("readonly"))||(tag == 'TEXTAREA' && !$(target).attr("readonly"))){
     if((target.type.toUpperCase() == "RADIO") || (target.type.toUpperCase() == "CHECKBOX")){
      return false ;
     }else{
      return true ; 
     }
    }else{
     return false ;
    }
   }
  });