textarea 自适应高度

textareaAdaptive.css

#textarea { 
	display: block;
	margin:0 auto;
	overflow: hidden; 
	width: 550px; 
	font-size: 14px;
	height: 18px; 
	line-height: 24px;
	padding:2px; 
}
textarea {
	outline: 0 none;
	border-color: rgba(82, 168, 236, 0.8);
	box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.1), 0 0 8px rgba(82, 168, 236, 0.6);
}

textareaAdaptive.js

/**
 * 文本框根据输入内容自适应高度
 * @param elem {HTMLElement}  输入框节点
 * @param extra {Number}  设置光标与输入框保持的距离（默认为0）
 * @param maxHeight {Number}  设置最大高度（可选)
 */
var autoTextarea = function (elem, extra, maxHeight) {
    extra = extra || 0;
    var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window;
    var isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera');
    var addEvent = function (type, callback) {
        elem.addEventListener ?
            elem.addEventListener(type, callback, false) :
            elem.attachEvent('on' + type, callback);
    };
    var 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];
    };
    var minHeight = parseFloat(getStyle('height'));

    elem.style.resize = 'none';

    var change = function () {
        var scrollTop, height, paddingTop, paddingBottom;
        var doc = document;
        var padding = 0;
        var style = elem.style;

        if (elem._length === elem.value.length) return;
        elem._length = elem.value.length;

        if (!isFirefox && !isOpera) {
            paddingTop = parseInt(getStyle('paddingTop'));
            paddingBottom = parseInt(getStyle('paddingBottom'))
            padding = paddingTop + paddingBottom;
        }

        scrollTop = doc.body.scrollTop || doc.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();
};