textarea自适应高度

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var autoTextarea = function (elem, extra, maxHeight) {
 
    extra = extra || 20;
 
    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.maxHeight = 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();
 
};