css代码：
<style type="text/css">
	 textarea{
	 	display: block;
	 	width: 500px;
	 	height: auto;
	 	margin:0 auto;
	    overflow: hidden; 
	    font-size: 14px;
	    height: 18px;
	    line-height: 24px;
	    padding:2px;
	 }
</style>

html代码：
<textarea id="textarea"></textarea>

js代码一：
<script type="text/javascript">
function autoHeight(el) {
    var timer = null;
    //由于ie8有溢出堆栈问题，故调整了这里
    var setStyle = function(el, auto) {
        if (auto) el.style.height = 'auto';
        el.style.height = el.scrollHeight + 'px';
    }
    var delayedResize = function(el) {
        if (timer) {
            clearTimeout(timer);
            timer = null;
        }
        timer = setTimeout(function() {
            setStyle(el)
        }, 200);
    }
    if (el.addEventListener) {
        el.addEventListener('input', function() {
            setStyle(el, 1);
        }, false);
        setStyle(el)
    } else if (el.attachEvent) {
        el.attachEvent('onpropertychange', function() {
            setStyle(el)
        })
        setStyle(el)
    }
    if (window.VBArray && window.addEventListener) { //IE9
        el.attachEvent("onkeydown", function() {
            var key = window.event.keyCode;
            if (key == 8 || key == 46) delayedResize(el);

        });
        el.attachEvent("oncut", function() {
            delayedResize(el);
        }); //处理粘贴
    }
}

var textarea = document.getElementById('textarea');
autoHeight(textarea);
</script>

js代码二：
<script type="text/javascript">
var autoTextarea = function (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();
};

var text = document.getElementById("textarea");
autoTextarea(text);
</script>

js代码三：
<script type="text/javascript">
$(function(){
    $.fn.autoHeight = function(){    
       function autoHeight(elem){
           elem.style.height = 'auto';
           elem.scrollTop = 0; //防抖动
           elem.style.height = elem.scrollHeight + 'px';
       }
       this.each(function(){
           autoHeight(this);
           $(this).on('keyup', function(){
               autoHeight(this);
           });
       });     
   }         
   $('#textarea').autoHeight(); 
})
</script>