karma.conf.js

{
  //...
  // plugins
  plugins: [
    'karma-*',
    {
      'middleware:custom': ['factory', function (config) {
        return function (req, res, next) {
          console.log('HELLO!')
          res.end('hello-world')
        }
      }]
    }
  ],
  // middlewares
  middlewares: ['custom'],
  //...
}