Character countdown like on twitter

Make a span and textarea and give them unique selectors (using an ID or class) like so:

<textarea class="message" rows="2" cols="30"></textarea>
<span class="countdown"></span>

And then make an update function like so:

function updateCountdown() {
    // 140 is the max message length
    var remaining = 140 - jQuery('.message').val().length;
    jQuery('.countdown').text(remaining + ' characters remaining.');
}

And make that function run when the page is loaded and whenever the message is changed:

jQuery(document).ready(function($) {
    updateCountdown();
    $('.message').change(updateCountdown);
    $('.message').keyup(updateCountdown);
});

Visit an example and view the source. jQuery makes things like this very simple.

I've used Aaron Russell's simply countable jQuery plugin with success; though, if I were to have written it, I would have designed it a bit differently (automatic counter div creation, using a data-maxlength attribute instead of a plugin option, etc).

Simple usage:

$('#my_textarea').simplyCountable();

Advanced usage:

$('#my_textarea').simplyCountable({
    counter: '#counter',
    countable: 'characters',
    maxCount: 140,
    strictMax: false,
    countDirection: 'down',
    safeClass: 'safe',
    overClass: 'over',
    thousandSeparator: ','
});