javascript验证textarea长度

1

<s2:textarea id="content" name="infoSingle.infoContent" rows="12" cols="85" onkeydown="check(content,ContentUse,ContentRem,500)" onkeyup="check(content,ContentUse,ContentRem,500)" onchange="check(content,ContentUse,ContentRem,500)"/>

 

2.

function check(fieldName,UseName,RemName,len){
    alert(fieldName.value);
    if(fieldName.value.length>len){
        fieldName.value=(fieldName.value).substring(0,len);
        alert("����ֻ�������� "+len+" ���ַ�");
        return false;
    }
    else{
        UseName.value=eval(fieldName.value.length);
        RemName.value=len-UseName.value;
        return true;
    }
}