maxlength.js

// See demo: http://codepen.io/kwarkjes/pen/VjYYqG

$('[data-maxlength]').each(function() {
    var $checker = $(this);
    var data = $checker.data('maxlength');

    $(data.selector).keyup(function() {
        var diff = parseInt(data.max - $(this).val().length);
        $checker.text(diff).css('color', diff < 1 ? 'red' : '');
    }).keyup();
});

/*
    HTML MARKUP
    <textarea id="social-message">Hello World!</textarea>
    <span data-maxlength='{"selector": "#social-message", "max": 140}'>140</span> of 140 characters
*/