how-middleware-pipeline-works.js

const MiddlewarePipeline = () => {
  const _middlewares = []

  return {
    apply: middleware => _middlewares.push(middleware),

    run: function run (middlewares) {
      if (!middlewares) {
        // set for first time call `run`
        middlewares = _middlewares
      }

      if (middlewares.length > 0) {
        const next = () => run(middlewares.slice(1))
        return middlewares[0](next)
      } else {
        return
      }
    }
  }
}

;(main => {
  const pip = MiddlewarePipeline()

  // say hi
  pip.apply(next => {
    console.log('hello!')
    next()
    console.log('goodbye.')
  })

  // just wow~
  pip.apply(next => {
    console.log('wow')
    next()
  })

  pip.run()
})()

outputs:

❯ node how-middleware-pipeline-works.js
hello!
wow
goodbye.