rottentweets.js

var ids = [...];
    rottenTweets;

_(ids).each(function(id) {
  getTweet(id, function(tweetBody) {
    rot13(tweetBody, function(rottenTweet) {
      rottenTweets.push(rottenTweet);
      
      if (rottenTweets.length == ids.length) {
        useRottenTweets();
      }
    });
  });
});

function useRottenTweets() {
  console.log(rottenTweets);
}