I alway do it like this:
$(document).ready(function(){
var maxChars = $("#sessionNum");
var max_length = maxChars.attr('maxlength');
if (max_length > 0) {
maxChars.on('keyup', function(e){
length = new Number(maxChars.val().length);
counter = max_length-length;
$("#sessionNum_counter").text(counter);
});
}
});
Input:
<input name="sessionNum" id="sessionNum" maxlength="5" type="text">
Number of chars: <span id="sessionNum_counter">5</span>