js实时提示文本框还能输入的字数

//实时提示文本框还能输入的字数
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>