这种写法为什么会报错？

Question

function attEvent(obj,ev,fn)
{
if(obj.addEventListener)
{
obj.addEventListener(ev,fn,false);
}
else if(obj.attachEvent)
{
obj.attachEvent('on'+ev,fn); //导致这里报错
}
}function throttle()
{

}attEvent(window,'scroll',function(){});//IE6下正常
attEvent(window,'scroll',throttle());//IE6下，导致上面报错，类型不匹
求指点.............

Answer 1

attEvent(window,'scroll',function(){});//IE6下正常
attEvent(window,'scroll',throttle);加括号是执行函数，绑定的是 throttle 函数的返回值。throttle 函数 中 return function(){};  就可以那样写没return , 返回值是 undefined ，当然报错