jQuery实现

(function($){$.fn.autoTextarea = function(options) {var defaults={maxHeight:null,//文本框是否自动撑高，默认：null，不自动撑高；如果自动撑高必须输入数值，该值作为文本框自动撑高的最大高度minHeight:$(this).height() //默认最小高度，也就是文本框最初的高度，当内容高度小于这个高度的时候，文本以这个高度显示};var opts = $.extend({},defaults,options);return $(this).each(function() {$(this).bind("paste cut keydown keyup focus blur",function(){var height,style=this.style;this.style.height =  opts.minHeight + 'px';if (this.scrollHeight > opts.minHeight) {if (opts.maxHeight && this.scrollHeight > opts.maxHeight) {height = opts.maxHeight;style.overflowY = 'scroll';} else {height = this.scrollHeight;style.overflowY = 'hidden';}style.height = height  + 'px';}});});};})(jQuery);

调用方式

$("#textarea_id").autoTextarea({maxHeight:220});  

js实现方式

/*** 文本框根据输入内容自适应高度**/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();};

调用方式

autoTextarea(document.getElementById('textarea_id'));