js实时提示文本框还能输入的字数

1. //实时提示文本框还能输入的字数  
2. function countChar(textareaName,spanName){  
3.         var val = document.getElementById(textareaName).value;  
4.          if(300-val.length <=0) {  
5.             document.getElementById(spanName).innerHTML = 0;  
6.             document.getElementById(textareaName).value = val.substring(0,300);  
7.          }else{  
8.             document.getElementById(spanName).innerHTML = 300-val.length;  
9.             document.getElementById("tiaoshu").innerHTML = Math.floor(val.length/71)+1;  
10.          }  
11.     }  
12.   
13. <textarea id="content" name="content"  
14.                     style="width: 550px; height: 200px;" onBlur="checkContent()" onkeydown='countChar("content","counter");' onkeyup='countChar("content","counter");'></textarea>