textarea高度自适应，随文字增加自动撑开

需求：随文字增多，将textarea自动撑开。

解决思路： pre标签能保持文本的格式，将textarea的文本内容和样式严格复制一份给pre标签，pre的高度值就为我们所需的textarea的高度值。然后将pre隐藏。

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<style>
  #fit1{
    width: 674px;
    margin: 0 0 30px 0;
    padding: 10px 10px 20px;
    border: 1px solid #e6e6e6;
    min-height: 88px;
    resize: none;
    line-height: 24px;
    border-radius: 3px;
    font-size: 14px;
    font-family: '微软雅黑', Helvetica, sans-serif;
    overflow: hidden;
    word-wrap: break-word;
    white-space: pre-wrap;
  }
  .hide{
    visibility: hidden;
    position: absolute;
    z-index: -100;
  }
  #fit2{
    width: 674px;
    padding: 10px 10px 20px;
    min-height: 88px;
    resize: none;
    border: 1px solid #e6e6e6;
    line-height: 24px;
    border-radius: 3px;
    font-size: 14px;
    font-family: '微软雅黑', Helvetica, sans-serif;
    overflow: hidden;
    word-wrap: break-word;
  }
</style>
<body>
<div>
  <pre id="fit1" class="hide"></pre>
  <textarea id="fit2"></textarea>
</div>
<script>
  //高度自适应
  document.onkeydown = function () {
    var textarea = document.getElementById('fit2')
    var pre = document.getElementById('fit1');
    pre.innerHTML = textarea.value;
    var realHeight = pre.offsetHeight;//offsetHeight = height + padding + border
    if(realHeight > 120) textarea.style.height = (realHeight + 24 -32) + 'px'; //加24为一行的高度，减32为padding和border
    else textarea.style.height = realHeight - 32 + 'px';
  }
</script>
</body>
</html>

方法二：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title>无标题文档</title>
    <style type="text/css">
        *{margin:0; padding:0;}
        .t{width:20em;}
        .t pre{width:20em;}
        pre{
            white-space:pre-wrap;
            word-wrap:break-word;
        }
        #divhidden{
            visibility:hidden;
        }
    </style>
</head>

<body>

    <textarea id="textareas" class="t"  onkeyup="autoTextarea()"> </textarea>
    <div class="t" id="divhidden"></div>
             <script language="javascript">
                function autoTextarea(){
                    var divhidden = document.getElementById("divhidden");
                    var textareas = document.getElementById("textareas");
                    var val = textareas.value;
                    divhidden.innerHTML ="<pre>" +val+" "+"</pre>";
                    textareas.style.height = (divhidden.offsetHeight-2) + "px";
                    if(val == ""){
                        textareas.value = " ";
                    }
                }
            </script>
</body>
</html>   

不知道这个作者是谁了，sorry~

感谢分享https://blog.csdn.net/ls32140/article/details/78925452

textarea高度自适应，随文字增加自动撑开

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