Limit textarea characters, maybe display remaining

I just noticed that I can put an unlimited amount of characters in my custom text areas. (oh oh)

I found where i need to put in a div for some javascript >>>> show_block_custom_item.php

Although this may be obvious to most programers, but where do I put my js file? And will a .html message display on the add to cart page?

I also found this plug online and wonder if it might be a better solution for what I'm looking to do. I don't see how because my plug specifically links with the textarea id.

Thanks in advance.
Lena

Not as easy as I thought it would be. Spinning my wheels again.

Not only did I want to limit the number of characters in a custom textarea (one of my textarea id's is "message"), I also wanted to show the users how many characters were remaining in the same window with the following js. This is what I meant by the html message on the add to cart page. I'm either not using proper syntax to add the necessary divs and/or there's another js language out there for joomla that doesn't jive with the js lessons i've been studying. The only way I can seriously be helped is (i think) to get explicit and detailed code of what to add where.


This "message.js" file was uploaded to mywebsite/media/hikashop/js and I edited the show_block_custom_item.php file to include several divs as well as $doc =& JFactory::getDocument();
$doc->addScript(" www.mywebsite.com/media//hikashop/message.js ") and I'm getting nowhere fast.

message.js below

var message_max = 310;

$('#hikashop_item_message_line').keyup(function() {
var message_length = $('#hikashop_item_message_line').val().length;
var char_left = message_max - message_length;
${'#message_feedback'}.html('You have ' + char_left + ' remaining');
});

I can't believe no one else needs these stuff ...

I sooooooooooo appreciate this program, and you!
Thank you.