实现select change 点击重复内容无法触发问题

var id = document.getElementById("addquerstions");
id.onmousedown = function(){
        this.sindex = this.selectedIndex;
        this.selectedIndex = -1;
    }
    id.onmouseout = function() {
        var index = id.selectedIndex;
        if (index == -1) {
            this.selectedIndex = this.sindex;
        }
    }