Introducing

The stylish Node.js middleware engine

for AWS Lambda

npm install middy

const middy = require('middy')
const { urlEncodedBodyParser, validator, httpErrorHandler } = require('middy/middlewares')

const processPaymentHandler = (event, context, callback) => {
  const { 
    creditCardNumber,
    expiryMonth,
    expiryYear,
    cvc,
    nameOnCard,
    amount
  } = event.body
  
  // do stuff with this data ...
  
  return callback(null,
    { result: 'success', message: 'payment processed correctly'}
  )
}

const inputSchema = {
 // define validation schema here ...
}

const handler = middy(processPaymentHandler)
  .use(urlEncodedBodyParser())
  .use(validator(inputSchema))
  .use(httpErrorHandler())

module.exports = { handler }

Why?

• Simplify code
• Reusability
  • input parsing
  • input & output validation
  • output serialization
  • error handling
  • ...
• Focus MORE on business logic

Usage

const middy = require('middy')
const { middleware1, middleware2, middleware3 } = require('middy/middlewares')

const originalHandler = (event, context, callback) => {
  /* your business logic */
}

const handler = middy(originalHandler)

handler
  .use(middleware1())
  .use(middleware2())
  .use(middleware3())

module.exports = { handler }

How it works

Execution order

1. middleware1 (before)
2. middleware2 (before)
3. middleware3 (before)
4. handler
5. middleware3 (after)
6. middleware2 (after)
7. middleware1 (after)

When an error happens...

• Flow is stopped
• First middleware implementing `onError` gets control
• It can choose to handle the error, or delegate it to the next handler
• If the error is handler a response is returned
• If the error is not handled the execution fails reporting the unhandled error

Writing a middleware

const myMiddleware = (config) => {
  
  // might set default options in config
  
  return ({
    before: (handler, next) => {
      // might read options from `config`
    },
    after: (handler, next) => {
      // might read options from `config`
    },
    onError: (handler, next) => {
      // might read options from `config`
    }
  })
}

module.exports = myMiddleware

Inline middlewares

const middy = require('middy')

const handler = middy((event, context, callback) => {
  // do stuff
})

handler.before((handler, next) => {
  // do something in the before phase
  next()
})

handler.after((handler, next) => {
  // do something in the after phase
  next()
})

handler.onError((handler, next) => {
  // do something in the on error phase
  next()
})

module.exports = { handler }

Currently availble middlewares

Json Body Parser

Url Encoded Body Parser

Validator

HTTP Error Handler

In development

That's all folks

(for now!)

https://github.com/Plnt9/middy