function(error, callback) {
  if (error) {
    return callback(error);
  }

  // If there is no error then do something
  // and then eventually return.
  return callback(null);
}

var async = require('async');

var db = {
  'foo': 'fizz',
  'bar': 'buzz',
};

function get(key, callback) {
  if (db[key]) {
    return callback(null, db[key]);
  }

  return callback('Key "' + key + '" Not Found');
}

function set(key, val, callback) {
  db[key] = val;

  return callback(null, db[key]);
}

async.waterfall([
  function(callback) {
    get('foo', function(error, foo) {
      if (error) {
        return callback(error);
      }

      return callback(null, foo);
    });
  },
  function(foo, callback) {
    get('bar', function(error, bar) {
      if (error) {
        return callback(error);
      }

      return callback(null, foo, bar);
    });
  },
  function(foo, bar, callback) {
    set('bar', foo, function(error, bar) {
      if (error) {
        return callback(error);
      }

      return callback(null, foo, bar);
    });
  },
], function(error, foo, bar) {
  if (error) {
    return console.log(error);
  }

  console.log(foo);
  console.log(bar);
});

function(foo, bar, callback) {
  if (foo !== bar) {
    set('bar', foo, function(error, bar) {
      if (error) {
        return callback(error);
      }

      return callback(null, foo, bar);
    });
  }

  return callback(null, foo, bar);
},

function(foo, bar, callback) {
  if (foo !== bar) {
    return set('bar', foo, function(error, bar) {
      if (error) {
        return callback(error);
      }

      return callback(null, foo, bar);
    });
  }

  return callback(null, foo, bar);
},

function(foo, bar, callback) {
  if (foo !== bar) {
    set('bar', foo, function(error, bar) {
      if (error) {
        return callback(error);
      }

      return callback(null, foo, bar);
    });
  } else {
    return callback(null, foo, bar);
  }
},