textarea字数限制

<script type="text/javascript">
function collect_submit() {
var len = $(".describe").val().length;
var num = 500 - len;
if (len > 501) {
$(this).val($(this).val().substring(0, 500));
}
if (len < 5) {
alert("最少5个字");
}
}

</script>

<textarea name="describe" class="describe" cols="54" rows="10"
onchange="this.value=this.value.substring(0, 500)"
onkeydown="this.value=this.value.substring(0, 500)"
onkeyup="this.value=this.value.substring(0, 500)"
placeholder="请尽可能详细的的描述您的业务，至少5个字，最多500个字">

</textarea>