@poppinss/middleware

Implementation of the chain of responsibility design pattern.

This package is a zero-dependency implementation for the chain of responsibility design pattern, also known as the middleware pipeline.

Setup

Install the package from the npm packages registry.

npm i @poppinss/middleware

# yarn lovers
yarn add @poppinss/middleware

And import the Middleware class as follows.

import Middleware from '@poppinss/middleware'
import { NextFn } from '@poppinss/middleware/types'

const context = {}
type MiddlewareFn = (
  ctx: typeof context,
  next: NextFn
) => void | Promise<void>

const middleware = new Middleware<MiddlewareFn>()

middleware.add((ctx, next) => {
  console.log('executing fn1')
  await next()
})

middleware.add((ctx, next) => {
  console.log('executing fn2')
  await next()
})

await middleware
  .runner()
  .run((fn, next) => fn(context, next))

Defining middleware

The middleware handlers are defined using the middleware.add method. The middleware function can be represented as any value you wish. For example:

The middleware can be a function

const middleware = new Middleware()

middleware.add(function () {
  console.log('called')
})

Or it can be an object with handle method

const middleware = new Middleware()
function authenticate() {}

middleware.add({ name: 'authenticate', handle: authenticate })

Passing data to middleware

Since, you are in control of executing the underlying middleware function. You can pass any data you want to the middleware.

const context = {}

type MiddlewareFn = (
  ctx: typeof context,
  next: NextFn
) => void | Promise<void>

const middleware = new Middleware<MiddlewareFn>()

middleware.add(function (ctx, next) {
  assert.deepEqual(ctx, context)
  await next()
})

const runner = middleware.runner()
await runner.run((fn, next) => fn(context, next))

Final Handler

The final handler is executed when the entire middleware chain ends by calling next. This makes it easier to execute custom functions that are not part of the chain but must be executed when it ends.

const context = {
  stack: [],
}

type MiddlewareFn = (
  ctx: typeof context,
  next: NextFn
) => void | Promise<void>

const middleware = new Middleware<MiddlewareFn>()

middleware.add((ctx: typeof context, next: NextFn) => {
  ctx.stack.push('fn1')
  await next()
})

await middleware
  .runner()
  .finalHandler(() => {
    context.stack.push('final handler')
  })
  .run((fn, next) => fn(context, next))

assert.deepEqual(context.stack, ['fn1', 'final handler'])

Error handler

By default, the exceptions raised in the middleware pipeline are bubbled upto the run method and you can capture them using try/catch block. Also, when an exception is raised, the middleware downstream logic will not run, unless middleware internally wraps the next method call inside try/catch block.

To simply the exception handling process, you can define a custom error handler to catch the exceptions and resume the downstream flow of middleware.

const context = {
  stack: [],
}

type MiddlewareFn = (ctx: typeof context, next: NextFn)
const middleware = new Middleware<MiddlewareFn>()

middleware.add((ctx: typeof context, next: NextFn) => {
  ctx.stack.push('middleware 1 upstream')
  await next()
  ctx.stack.push('middleware 1 downstream')
})

middleware.add((ctx: typeof context, next: NextFn) => {
  ctx.stack.push('middleware 2 upstream')
  throw new Error('Something went wrong')
})

middleware.add((ctx: typeof context, next: NextFn) => {
  ctx.stack.push('middleware 3 upstream')
  await next()
  ctx.stack.push('middleware 3 downstream')
})

await middleware
  .runner()
  .errorHandler((error) => {
    console.log(error)
    context.stack.push('error handler')
  })
  .finalHandler(() => {
    context.stack.push('final handler')
  })
  .run((fn, next) => fn(context, next))

assert.deepEqual(context.stack, [
  'middleware 1 upstream',
  'middleware 2 upstream',
  'error handler',
  'middleware 1 downstream'
])