JavaScript即時提示輸入框輸入的字數

JavaScript即時提示輸入框輸入的字數，如下代碼：

 

  
//即時提示文字框還能輸入的字數   
  
function countChar(textareaName,spanName){   
  
        var val = document.getElementById(textareaName).value;   
  
         if(300-val.length <=0) {   
  
            document.getElementById(spanName).innerHTML = 0;   
  
            document.getElementById(textareaName).value = val.substring(0,300);   
  
         }else{   
  
            document.getElementById(spanName).innerHTML = 300-val.length;   
  
            document.getElementById("tiaoshu").innerHTML = Math.floor(val.length/71)+1;   
  
         }   
  
    }   
  
   
  
<textarea id="content" name="content"   
  
                    style="width: 550px; height: 200px;" onBlur="checkContent()" onkeydown='countChar("content","counter");' onkeyup='countChar("content","counter");'></textarea>