计算可以输入字数 用法

1.第一种

div id="mod_order_remark" style="display:none;">
  <div class="write_mod">
    <div class="wmhd"> 
     <span class="order_id">
       <strong>订单号:</strong>1234567890
     </span>
     <span class="order_create_time">
        <strong>下单时间：</strong>2010-08-07 11:52</span>
        <span class="trade_company">企业：
           <a href="#" title="#">上海三星数码我友专供</a>
        <span class="btn_d">
          <input type="button" class="input_bt" value="联系我"/>
        </span>
     </span>
    </div>
    <div class="wmbd">
      <h3 class="mod_title">添加/编辑备注</h3>
      <textarea class="mod_content" name="remork" 
       id="remork" onkeydown='countChar("status","counter");'      onkeyup='countChar("remork","counter",this);'>
      </textarea>
      <span class="letter_sum">剩余字数：
       <span id="counter">250</span>/250字</span>
      <div class="return">
        <span class="btn_a">
        <input type="button" class="input_bt" 
         id="btnConfirm" value="确定" />
        </span>
        <span class="btn_a">
           <input type="button" class="input_bt" 
            id="cancelBtn" value="取消" />
        </span>
      </div>
    </div>
  </div>
</div>

引用

function countChar(textareaName,spanName,field)
{  
maxlimit=250;
document.getElementById(spanName).innerHTML = maxlimit - document.getElementById(textareaName).value.length;

    if (field.value.length > maxlimit)
    field.value = field.value.substring(0, maxlimit);
}

2.第二种

<script>   
function cal() {   
  if(a.value.length>20) {   
    a.value=a.value.slice(0,20);   
    b.value=20;   
    c.value=0;   
    }   
  else {   
    b.value=a.value.length;   
    c.value=20-eval(b.value);   
    }   
  }   
</script>   
<textarea id="a" onpropertychange="cal()"></textarea>   
已有字数：<input id=b size=5 value="0" disabled>   
剩余字数：<input id=c size=5 value="20" disabled><br>