textarea 自动撑开高度

var textAreaArr = document.querySelectorAll('.textarea');
    for (var i = 0; i < textAreaArr.length; i++) {
    autoTextarea(textAreaArr[i]);
}
function autoTextarea(elem, extra, maxHeight) {
        extra = extra || 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 (maxHeight && elem.scrollHeight > maxHeight) {
                                height = maxHeight - padding;
                                style.overflowY = 'auto';
                        } else {
                                height = elem.scrollHeight - padding;
                                style.overflowY = 'hidden';
                        }
                    style.height = height + extra + 'px';
                        scrollTop += parseInt(style.height) - elem.currHeight;
                        document.body.scrollTop = scrollTop;
                        document.documentElement.scrollTop = scrollTop;
                        elem.currHeight = parseInt(style.height);
                }
        };
 
        addEvent('propertychange', change);
        addEvent('input', change);
        addEvent('focus', change);
        change();
    }