初学面向对象javascript，写程序如下（目的是使一个Div高度慢慢缩短，最后消失）：

var animateObj = function() {
    this.animateTimer = "";
    this.intervalTime = 1;
    this.divEl = null;
    this.divHeight = 0;
    this.shortElRegularly = function() {
        if (divHeight > 0) {
            divEl.style.height = divHeight.toString() + "px";
            divHeight--;
        }
        else {
            clearInterval(animateTimer);
            hide(divEl);
            divEl.style.removeAttribute("height");
        }
    }
    this.startShortAnimate = function(el) {
        divEl = el;
        divHeight = divEl.scrollHeight;
        alert('1');
        animateTimer = setInterval(this.shortElRegularly, intervalTime);
        alert('2');
    }
}
function cutDisplay(el) {
    if (!isHide(el)) {
        var animateObjInstance = new animateObj();
        animateObjInstance.startShortAnimate(el);
    }
}

调用后只能执行到alert('1');执行不知alert('2');

请问setInterval应该怎么写啊？还有哪些地方写的不合适的？请大家赐教。。。