A Pen by Ken Shoufer.

index.html

<h2>Tweet Box</h2>
<textarea name="" id="tweet-box" cols="40" rows="6"></textarea>
<p>Characters Remaining: <span id="tweet-feedback"></span></p>

script.js

var tweetMax = 140;
var tweetValue;
var tweetRemaining;

$('#tweet-box').keyup(function() {
    tweetValue = $('#tweet-box').val().length;
    tweetRemaining = tweetMax - tweetValue;
    if (tweetRemaining < 0) {
      $('#tweet-feedback').addClass("overLimit");
    } else {
      $('#tweet-feedback').removeClass("overLimit");
    }

    $('#tweet-feedback').html(tweetRemaining);
});

style.css

#tweet-feedback {
   color: #333;
   background-color: #EFD994;
   padding: 10px;
   text-align: center;
}

#tweet-feedback.overLimit {
    color: #fff;
    background-color: #EE3370;
}

Tweet-Box.markdown

Tweet Box

Example of a javascript callback.

A Pen by Ken Shoufer on CodePen.

License.