textarea validation using javascript

Below is the code which can be used to limit the number of characters in a text area using javascript.

File 1: limittext.js

function limitText(limitField, limitCount, limitNum) {
    if (limitField.value.length > limitNum) {
        limitField.value = limitField.value.substring(0, limitNum);
    } else {
        limitCount.value = limitNum - limitField.value.length;
    }
}
File 2: index.html

<table>

<tr>
            <td  class="text_heading"><strong>Address1 </strong></td>

            <td colspan="3"  align="left"  class="normal">

        <textarea id="addr1" name="addr1"  onKeyDown="limitText(this.form.addr1,this.form.countdown1,500);"

onKeyUp="limitText(this.form.addr1,this.form.countdown1,500);" maxlength="500"><%=BANK_ADDRESS1%></textarea>
             <br>
<font size="1">(Maximum characters: 500)<br>
You have <input readonly type="text" name="countdown1" size="3" value="500"> characters left.</font>     </td>
  </tr>
</table>