html+css+Jquery 实现文本域文字数量限制、根据输入字数自适应高度

先看效果：初始的效果，样式多少有点问题，不重要！重要的是功能！

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输入后：根据文字长度，决定文本域长度 + 限制文字数量

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话不多说，直接上代码！

<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<title></title>
</head>
<style type="text/css">
.foot-send-img img {
	width: 20px;
	height: 20px;
}
.text-input_5ZOdm{
  display: flex;
  justify-content: center;
  align-items: end;
  position: relative;
  box-sizing: border-box;
  font-family: Arial,Helvetica,sans-serif;
  width: 649px;
  height: 40px;
  background: #FFFFFF;
  box-shadow: 0px 0px 6px 1px #ee1903;
  border-radius: 10px;
}
.text-container{
  width: 100%;
  height:100%;
}
.text-input-textarea_chat{
  background: #FFFFFF;
  line-height: 28px;
  border-radius: 10px;
  resize: none;
  outline-color: #ffff;
  width: 100%;
  height: 100%;
  border: none;
  outline: none;
  word-break: break-word;
  font-size: 16px;
  overflow: hidden;
  padding-left: 14px;
  padding-top: 7px;
}
.chat-number{
  float: right;
  position: relative;
  font-size: 12px;
  font-family: PingFangSC, PingFang SC;
  font-weight: 400;
  color: #D1D1D1;
  bottom: 22px;
  margin-right: 10px;
}
.foot-send-img{
  width: 55px;
  background: #FFA245;
  border-radius: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  height: 34px;
  bottom: 3px;
  position: relative;
}
.sidebar_right-left{
	    display: flex;
    align-items: center;
    padding-left: 76px;
}
</style>
<body>
<div class="sidebar_right-left">
	<div class="text-input_5ZOdm  text-enable_3rWFc"> 
  <div class="text-container">
    <textarea class="text-input-textarea_chat chat-input-size" id="chat-input-size" placeholder="请问我 您想了解的问题" maxlength="50"></textarea>
      <p class="chat-number"><span id="textNum">0</span>/<span>50</span></p>
  </div>

  <div class="foot-send-img" id="send-btn">
    <img src="./images/img1.jpg" align="middle"  class="chat-foot-send">
  </div>
</div>
</div>
</body>
<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script type="text/javascript">
	
$("#chat-input-size").on('focus input',function(){
  var textarea = $('#chat-input-size');
  computerTextareaRows(textarea);
  var text = textarea.val();

  var counter = text.length;
  if (text.indexOf("请问我您想了解的问题") > -1){
    textarea.val('')
  }
  $("#textNum").text(counter);
  if(counter >= 50){
    $('.chat-number').css('color','#FF7F7F');
    return false;
  }else{
    $('.chat-number').css('color','#D1D1D1');
  }
}).on('blur',function(){
  var textarea = $('#chat-input-size');
  var text = textarea.val();
  if (text === ''){
    textarea.val('请问我您想了解的问题')
  }
})

//根据texarea行数设置高度
function computerTextareaRows(textarea) {
  var lineHeight = parseFloat(textarea.css('line-height'));
  var paddingTop = parseFloat(textarea.css('padding-top'));
  var paddingBottom = parseFloat(textarea.css('padding-bottom'));

// 计算行数
  var contentHeight = textarea[0].scrollHeight - paddingTop - paddingBottom;
  let proportion = contentHeight / lineHeight;
  var rows = proportion < 2 ? 1 : Math.ceil(contentHeight / lineHeight);
  // console.log(rows,textarea[0].scrollHeight,proportion)
  if (rows>1){
    $('#chat-input-size').css('height','40px');
    $('.text-input_5ZOdm').css('height', 'auto');
    $('.text-input_5ZOdm').css('height', textarea[0].scrollHeight+'px');
    $('#chat-input-size').css('height',textarea[0].scrollHeight+'px');
    // $('#send-btn').css('margin-top','20px');
  } else {
    $('.text-input_5ZOdm').css('height','40px');
    // $('#send-btn').css('margin-top','0px');
  }
  return rows;
}
</script>
</html>