payback.js

$(document).ready(function(){

  // required vars    
    var accessToken = window.location.hash.split("=")[1];
    var userID = accessToken.split(".")[0];

  // step 1: get users followed
    getFollowedUsers(userID, accessToken, function(followed_users){

      // step 2a: check for non-mutual follows
        var completed_checks = 0;
        var total_nonmutuals = 0;
        var nonmutual_follows = [];

        $.each(followed_users, function(index, followed_user){

          isFollowershipMutual(followed_user.id, accessToken, function(is_mutual){

            if(!is_mutual){

              // step 2b: get metadata of non-mutual follows
                total_nonmutuals += 1;
                
                getUserMetadata(followed_user.id, accessToken, function(user_data){
                  
                  // step 2c: render markup for non-mutual follow
                    var username = user_data.username;
                    var profile = user_data.profile_picture;
                  
                  nonmutual_follows.push(
                    '<li class="unfollower" data-unfollowerId="'+followed_user.id+'">' +
                      '<a href="https://www.instagram.com/'+username+'" target="_blank">' +
                        '<img src="'+profile+'" alt="'+username+'">' +
                        '<span>'+username+'</span>' +
                      '</a>' +
                    '</li>'
                  );
                });
            }

            completed_checks += 1;
          })
        });

      // step 3: render list of nonmutual follows on an interval
        var rendered_nonmutuals = 0;
        var render_checker = setInterval(render_nonmutual_followers, 1000);

        function render_nonmutual_followers(){

          var markup = nonmutual_follows.join('');

          $('#payback').append( markup );
          $('#unfollowers-count').html(rendered_nonmutuals+" people don't follow back");

          rendered_nonmutuals += nonmutual_follows.length;
          nonmutual_follows = [];

          if(cancel_render_checker() === true){

            clearInterval( render_checker );
          } 
        }

        function cancel_render_checker(){

          if(completed_checks < followed_users.length) return false;

          if(rendered_nonmutuals < total_nonmutuals) return false;

          return true;
        }
    });

  function getFollowedUsers(userID, accessToken, callback){

    var api_endpoint = 'https://api.instagram.com/v1/users/'+userID+'/follows?access_token='+accessToken+'&count=500&callback=?';

    $.getJSON(api_endpoint, {}, function(response){

       callback( response.data );
    });
  }

  function isFollowershipMutual(followerID, accessToken, callback){

    var api_endpoint = 'https://api.instagram.com/v1/users/'+followerID+'/relationship?access_token='+accessToken+'&callback=?';

    $.getJSON(api_endpoint, {}, function(response){

      var is_mutual = (response.data.incoming_status !== 'none' ? true : false);

      callback( is_mutual );
    });
  }

  function getUserMetadata(userID, accessToken, callback){

    var api_endpoint = 'https://api.instagram.com/v1/users/'+userID+'/?access_token='+accessToken+'&callback=?';

    $.getJSON(api_endpoint, {}, function(response){

      var user_metadata = response.data;

      callback( user_metadata );
    })
  }
});

Fixed a couple of typos