js实现textarea自适应高度

html结构：

<div class="container" id="container">
    <div class="text-wrapper">
        <pre></pre>
        <textarea placeholder="请输入内容"></textarea>
    </div>
</div>

原理：使用 pre（可以保留空格和换行）元素保存 textarea 的内容，使得父元素 div 的高度撑高，再设置 textarea 的高度为 100%即可

css样式：

*,
*:before,
*:after {
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
}

body {
    font-size: 14px;
    font-family: '微软雅黑',"Helvetica Neue", Helvetica, Arial, sans-serif;
    color: #333;
    background-color: #fff;
    line-height: 1.6;
}

body, div, pre, textarea {
    margin: 0;
    padding: 0;
}

.container {
    width: 800px;
    margin: 20px auto;
}

.text-wrapper {
    position: relative;
    margin-bottom: 20px;
}

.text-wrapper pre {
    display: block;
    visibility: hidden;
    width: 100%;
    min-height: 40px;
    padding: 8px 10px;
    font-size: 14px;
    line-height: 1.6;
    border: 1px solid #aed0ea;
    white-space: pre-wrap;
    word-break: break-all;
    word-wrap: break-word;
}

.text-wrapper textarea {
    position: absolute;
    top: 0;
    left: 0;
    display: inline-block;
    width: 100%;
    height: 100%;
    box-sizing: border-box;
    padding: 8px 10px;
    font-size: 14px;
    line-height: 1.6;
    vertical-align: middle;
    background-color: #fff;
    border: 1px solid #aed0ea;
    border-radius: 4px;
    overflow: hidden;
    white-space: pre-wrap;
    word-break: break-all;
    word-wrap: break-word;
    resize: none;
    transition: border-color ease-in-out .15s, box-shadow ease-in-out .15s;
    -webkit-transition: border-color ease-in-out .15s, -webkit-box-shadow ease-in-out .15s;
    -moz-transition: border-color ease-in-out .15s, box-shadow ease-in-out .15s;
}

.text-wrapper textarea:focus {
    border-color: #66afe9;
    outline: 0;
    box-shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px rgba(102, 175, 233, 0.6);
    -webkit-box-shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px rgba(102, 175, 233, 0.6);
    -moz-box-shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px rgba(102, 175, 233, 0.6);
}

js代码：

var laoq = (function(){
    var addHandler = function(ele,type,fn){
        if(ele.addEventListener){
            ele.addEventListener(type,fn,false);
        }else if(ele.attachEvent){
            ele.attachEvent('on' + type,fn);
        }else{
            ele['on' + type] = fn;
        }
    };
    
    var setTextareaAutoHeight = function(){
        addHandler(document.getElementById('container'),'input',function(){
            var event = event || window.event,
                target = event.target || event.srcElement;
            
            if(target.type.toLowerCase() !== 'textarea')    return;
            
            var preele = target.previousElementSibling;
            
            preele && preele.tagName.toLowerCase() === 'pre' && target.parentElement.classList.contains('text-wrapper') && (preele.innerText = target.value);
        });
    };
    
    return{
        setTextareaAutoHeight:setTextareaAutoHeight
    };
})();

调用方式：

window.onload = function(){
    laoq.setTextareaAutoHeight();
};