请问为什么我把onmousemove事件写到function cutday里面再执行就会失效啊。下面那个tr2是可以正常执行的。上面的tr1写进函数里就不能执行。会报错。新人不懂。。求指点

<script type="text/javascript">
function cutday(a,b){
        var a = document.getElementById('a');
        a.onclick = function(){
            var tr_list = document.getElementsByClassName('tr_list');
            for(var i = 0 ; i < tr_list.length ; i++){
                tr_list[i].style.display = 'none'
            }
            document.getElementById('b').style.display = 'block'
        };
    }
        var tr2 = document.getElementById('tr2');
        tr2.onmousemove = function(){
            var tr_list = document.getElementsByClassName('tr_list');
            for(var i = 0 ; i < tr_list.length ; i++){
                tr_list[i].style.display = 'none'
            }
            document.getElementById('tr_pic2').style.display = 'block'
        };
        cutday(tr1,tr_pic1)
       
</script>