js仿微博输入框字数检测

/*
暂时不支持鼠标复制检测，但可以通过设置定时器或者blur的时候检测
*/
function TextNumCheck(elem) {
    this.elem = document.getElementById(elem);
}
TextNumCheck.prototype = {
    //跨浏览器获取文本
    text:function (obj, string) {
        return obj.innerText ? obj.innerText = string : obj.textContent = string;
    },
    //动画变色
    animBg:function (obj) {
        setTimeout(function () {
            obj.style.backgroundColor = "#fe9900";
            setTimeout(function () {
                obj.style.backgroundColor = "#fff";
            }, 150);
        }, 250);
    },
    /**
     * /检测已经输入多少字符
     * @param alrElem 要显示输入多少字符的元素id
     * @return {*}
     */
    curNum:function (alrElem) {
        var obj = document.getElementById(alrElem);
        var s = this.elem.value.length;
        this.text(obj, s);
        return this;
    },
    /**
     * 检测剩余多少字符
     * @param remainELem 要显示剩余字符的元素id
     * @param maxNum
     */
    numRemain:function (remainELem, maxNum) {
        var obj = document.getElementById(remainELem);
        var _thisElem = this.elem;
        var s = _thisElem.value.length;
        if (typeof maxNum === "undefined") {
            maxNum = 500;
        }
        var remain = maxNum - s;
        if (remain < 0) {
            _thisElem.value = _thisElem.value.substring(0, maxNum);
            this.animBg(_thisElem);
        }
        this.text(obj, remain);
        return this; 
    }
};
var txtNumCheck = new TextNumCheck("cErr_con");
document.getElementById("cErr_con").onkeyup = function () {
    txtNumCheck.curNum("lim_alreay").numRemain("lim_remain");
};