jsp页面屏蔽输入框只读属性时backspace返回上一级页面的方法

window.onload=function(){
document.getElementsByTagName("body")[0].onkeydown =function(){

var elem = event.relatedTarget || event.srcElement || event.target ||event.currentTarget;
//判断按键为backSpace键
if(event.keyCode==8){
var elem = event.srcElement || event.currentTarget;

var name = elem.nodeName;

if(name!='input' && name!='textarea'){
return _stopIt(event);
}
var type_e = elem.type.toUpperCase();
if(name=='INPUT' && (type_e!='text' && type_e!='textarea' && type_e!='password'
&& type_e!='file')){
return _stopIt(event);
}
if(name=='input' && (elem.readOnly==true || elem.disabled ==true)){
return _stopIt(event);
}
}
}
}

function _stopIt(e){
if(e.returnValue){
e.returnValue = false ;
}
if(e.preventDefault ){
e.preventDefault();
}

return false;
}