update.js

var main = function() {
    $('.btn').click(function() {
        var post = $('.status-box').val();
        $('<li>').text(post).prependTo('.posts');
        $('.status-box').val('');
        $('.counter').text('140');
    });
    $('.status-box').keyup(function(){
       var postLength = $(this).val().length;
       var charactersLeft = 140 - postLength;
       $('.counter').text(charactersLeft);
       if(charactersLeft < 0) {
            $('.btn').addClass('disabled');
        }
        else if(charactersLeft === 140){
            $('.btn').addClass('disabled');
        }
        else{
            $('.btn').removeClass('disabled');
        }
    });
    $('.btn').addClass('disabled');
};

$(document).ready(main);