textarea高度自适应js

有时候我们需要textarea高度随着行数多少进行跟随改变，而不是出现滚动条，以下方法和新浪微博的发布框一模一样效果，本人已用在项目中，分享出来给大家

function autoTextarea(options) {
    var defaults={
        $elem:'',            //jquery对象textarea
        extra:'',            //额外增加高度
        maxHeight:'',        //最大高度，默认无限
        callback:function($this,opts){}//高度改变之后的回调
    };
    var opts=$.extend({}, defaults , options || {});
    opts.extra = opts.extra || 0;
    var elem=opts.$elem[0];
    var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
        isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
        addEvent = function (type, callback) {
            elem.addEventListener ?
            elem.addEventListener(type, callback, false) :
            elem.attachEvent('on' + type, callback);
        },
        getStyle = elem.currentStyle ? function (name) {
            var val = elem.currentStyle[name];
            if (name === 'height' && val.search(/px/i) !== 1) {
                var rect = elem.getBoundingClientRect();
                return rect.bottom - rect.top - parseFloat(getStyle('paddingTop')) - parseFloat(getStyle('paddingBottom')) + 'px';
            };
            return val;
        } : function (name) {
            return getComputedStyle(elem, null)[name];
        },
        minHeight = parseFloat(getStyle('height'));

    elem.style.resize = 'none';
    var change = function () {
        var scrollTop, height,  padding = 0,  style = elem.style;
        if (elem._length === elem.value.length) return;
        elem._length = elem.value.length;
        if (!isFirefox && !isOpera) {
            padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
        };
        scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
        elem.style.height = minHeight + 'px';
        if (elem.scrollHeight > minHeight) {
            if (opts.maxHeight && elem.scrollHeight > opts.maxHeight) {
                height = opts.maxHeight - padding;
                style.overflowY = 'auto';
            } else {
                height = elem.scrollHeight - padding;
                style.overflowY = 'hidden';
            };
            style.height = height + opts.extra + 'px';
            scrollTop += parseInt(style.height) - elem.currHeight;
            document.body.scrollTop = scrollTop;
            document.documentElement.scrollTop = scrollTop;
            elem.currHeight = parseInt(style.height);
        };
        if(typeof opts.callback=='function'){
            opts.callback($(elem),opts);
        }
    };
    addEvent('propertychange', change);
    addEvent('input', change);
    addEvent('focus', change);
    change();
}