gistfile1.js

const final_handler = (request) => {
  return "hi!"
}

const a = (handler) => {  // handler is `b`
  return async (request) => {  
    return handler(request)
  }
}

const b = (handler) => {   // handler is `final_handler`
  return async (request) => { 
    return handler(request)
  }
}

(final_handler, [a, b]) // a gets called first, passing b as it's handler