Limit textarea alert message

I have the below that limits the textarea input to 500 characters but
cant get the alert message to work.  It doesnt show anything.  Please
advise.
<script language="javascript" type="text/javascript">
function limitText(limitField, limitCount, limitNum) {
	if (limitField.value.length > limitNum) {
		limitField.value = limitField.value.substring(0, limitNum);
                                alert("You are trying to enter more
than 500 characters");
	} else {
		limitCount.value = limitNum - limitField.value.length;
	}
}
</script>
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<form name="myform">
<textarea name="limitedtextarea"
onKeyDown="limitText(this.form.limitedtextarea,this.form.countdown,
500);"
onKeyUp="limitText(this.form.limitedtextarea,this.form.countdown,
500);">
</textarea>