test.js

// mw.js
var mw = function (options) {
  return function *(next) {
    this.db = options.db;
    yield next;
  }
}



// server.js
var mw = require('mw');
var db = require('memdb');

module.exports = function (app) {
  app.use(bodyParser());

  app.use(mw({
    db: db;
  }))

  app.use(_.get('/cats/:ids', function *(ids) {
    try {
      var data = yield ids.split(',').map(function (id) {
          return this.db.get(id);
      });
    } catch (e) {
      console.log('ERROR');
    }

    this.type = 'application/vnd.api+json';
    this.status = 200;
    this.body = JSON.stringify(data.length > 1 ? { body: data } : data[0]);
  }));

  return app;
}

// test.js
test('fetching a cat', function (t) {
  var app = koa();

  app.use(mw({
    db: {
      get: function (id) {
        var o = { bar: { name: 'mici' id: 'bar' } };
        return [ o[id] ];
      }
    }
  }));

  catDB(app);

  request(app.listen())
    .get('/cats/bar')
    .expect('Content-Type', 'application/vnd.api+json')
    .end(function (err, res) {
      if (err) t.end(err);

      t.equals(200, res.status);

      t.same(JSON.parse(res.text), { name: 'mici', id: 'bar' });
      t.end();
    });
});