Next.js Middleware Chain

Next.js Middleware Chain (NMC) is a lightweight, zero-dependency next.js middleware solution. It is built on the idea that middleware functions are more flexible when they can be chained as needed during development.

NMC aims to provide a simple framework for writing methods that can be used with either Next.js API routes or SSR (getServerSideProps) data fetching.

This library is still in an early stage of development. It has been tested it in a couple projects and should be stable, however there may be bugs or scenarios that have not been realized or planned for.

If you encounter a bug or problem, please open an issue.

If you have feedback or questions, please start a discussion.

Please be civil and abide by the code of conduct.

Why

Next.js does a lot of great things. Middleware is not one of them. If you want to implement some functionality for all API routes (logging at start & finish, for instance) you have to either explicitly add it to all your routes or write your own reusable solution.

This gets doubly complex if you want to re-use the same functionality across API Routes and SSR data fetching methods, since the function APIs and behaviors differ in small but important ways.

It would be a lot easier to just write the middleware functions you want and then compose them however you need when building out new pages and routes.

That's the goal.

Installation

npm install nextjs-middleware-chain

Setup & Usage

This package exports a single named function createMiddleware which takes two arguments, an array of middleware functions and an object of options to set globally for all middlware instances.

createMiddleware returns a factory function which, when called, returns an instance of a Middleware class which is used to run any subsequently chained functions.

This README will go into more detail for each part, but some initial context the setup & usage of NCM looks like:

// Setup in a `middleware.js` file...
import { createMiddleware } from 'nextjs-middleware-chain'

const mwFunction = (req, res, next) => {
  console.log('Running function!')

  return next()
}

const middlewareFunctionsArray = [ mwFunction ]

const optionsObject = { useChainOrder: true }

const mwFactory = createMiddleware(
  middlewareFunctionsArray,
  optionsObject
)

export default mwFactory

// Usage in an API or SSR route...
import mwFactory from './middleware'

/* ... api or ssr routeFunction(...) {...} ... */

export default mwFactory().mwFunction().finish(
  routeFunction,        // required
  'Friendly Route Name' // optional
)

This repo also includes a Next.js sample "client" project which you can reference & run to test out the library. You can reference the setup & syntax for NMC in these files:

• middleware setup
• api route implementation
• ssr route implementation

Middleware Functions Array

Any middleware functions you create will be made available to NMC by adding them to this array, and passing this array as the first argument to createMiddleware.

Options object

The default options will apply globally unless they are overwritten. You can overwrite options:

• globally: by passing an object as the second argument to the createMiddleware function.
• per-route: by passing an object as the only argument to the middleware factory responsible for creating an instance of the middleware.
  • mwFactory(optionsObject).mwFunction().finish(...)

Property	Type	Default	Purpose
useChainOrder	bool	true	Run middleware functions in the order they are used in the chain. When false this will always run the middleware function in the same order - the order they were added to the functions array passed to createMiddleware.
useAsyncMiddleware	bool	true	Run middleware functions asynchronously. When false middleware functions will run synchronously.
reqPropName	string	nmc	The name of the decoration property added to the req object for all routes. You can rename this if it causes a collision or of you don't like these letters.
onMiddlewareStart	function	(req) => {}	Run before starting the middleware chain. Receives the decorated req object as an argument.
onMiddlewareComplete	function	(req) => {}	Run after all middleware is complete (or when next() is called with 'end' or 'route') and before the Next.js route is run. Receives the decorated req object as an argument.
onRouteComplete	function	(req) => {}	Runs after the Next.js route has been run (or skipped). Receives the decorated req object as an argument.

Writing middleware functions

All middleware functions will receive 3 arguments:

• req
• res
• next

req and res are provided by the Next.js route. They will be passed by reference to all middleware functions, so can be mutated throughout the chain. Be aware than if you are using a chain function in both API and SSR contexts you may not have access to certain Next.js-provided decorations, which are only added for API routes:

• Request Helpers
• Response Helpers

next is a custom function for NMC. Each middleware function must return next() in order to continue running the middleware.

A basic middleware function looks like:

const middlewareChainFn = (req, res, next) => {
  // Do things here...
  
  return next()
}

Short-circuiting the middleware runner

You can break the chain in one of three ways.

To skip any remaining middleware functions and execute the Next.js route:

• return next('route')

To skip any remaining middleware function and ignore the Next.js route:

• From API routes: End the response via the res object, and return nothing.
  • res.status(###).json({ message: '...' })
• From SSR routes: Return with the keyword 'end' and a payload object.
  • return next('end', { ...payload })
  • The payload object is required - it will be the return from getServerSideProps so should be a standard Next.js getServerSideProps response.

const isAuthorized = (req, res, next) => {
  // Some authorization function...
  const authorized = getAuthorization(req)

  if (req.nmc.type === 'api' && !authorized) {
    // End the request if it is an API route
    res.status(401).json({error: 'unauthorized', message: '...'})
  } else if (req.nmc.type === 'ssr' && !authorized) {
    // End the request if it is an SSR route
    return next('end', {
      redirect: {
        destination: '/',
        permanent: false,
      }
    })
  } else {
    // Skip remaining middleware and continue with the Next.js route.
    return next('route')
  }
}

Request Decoration

NMC will add a custom property, nmc, (the prop name is configurable in options) to the req received by each middleware function. This object will contain:

Property	Type	Value
id	string	a random guid unique to each middleware instance
name	string	finalFuncName if provided, otherwise finalFunc.name, otherwise ''
type	string	api or ssr - the type of route the middleware is being run from*
context	object	The Next.js context object provided to SSR routes. Will be {} for API routes.

^* This determination is made based on the arguments the middleware runner receives from Next.js, which will either be (req, res) => {...} for API routes, or (context, undefined) => {...} for SSR routes.

Middleware Patterns

One Middleware to run Multiple Middlewares

You can run middleware functions yourself, if desired, but you will have to handle the returns from each as needed for your use-case:

const common = async (req, res, next) => {
  const aReturn = fnA(req, res, next)
  const bReturn = await fnB(req, res, next)
  const authReturn = unauthorized(req, res, next)

  if (authReturn) {
    return authReturn
  }
}

Prebuilt Middleware Chain

You can create pre-built chains of middleware if certain functions are commonly used together, or should always be run in a specific order:

// Creating a pre-built chain in middleware.js
const mwFactory = createMiddleware(
  middlewareFunctionsArray,
  optionsObject
)

const preBuiltChain = mwFactory().fnA().fnB().fnC()

export { preBuiltChain }
export default mwFactory

// Using a pre-built chain
import { preBuiltChain } from 'nextjs-middleware-chain'

/* ... api or ssr routeFunction(...) {...} ... */

export default preBuiltChain.otherMiddleware().finish(routeFunction)

Note that when you import & use the preBuiltChain it is already an instance of Middleware and so should be accessed as an object, not run as a method.

Planned Work

• Add automatic versioning & releases
• Add additional unit tests
• Move to TypeScript