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πŸ›΅ The stylish Node.js middleware engine for AWS Lambda
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Middy logo

The stylish Node.js middleware engine for AWS Lambda


A little appetizer

Middy is a very simple middleware engine. If you are used to web frameworks like express, than you will be familiar with the concepts adopted in Middy and you will be able to get started very quickly.

But code is better than 10,000 words, so let's jump into an example. Let's assume you are building a JSON API to process a payment:

# handler.js

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

// This is your common handler, in no way different than what you are used to doing every day
// in AWS Lambda
const processPayment = (event, context, callback) => {
  // we don't need to deserialize the body ourself as a middleware will be used to do that
  const { creditCardNumber, expiryMonth, expiryYear, cvc, nameOnCard, amount } = event.body

  // do stuff with this data
  // ...

  return callback(null, { result: 'success', message: 'payment processed correctly'})

// Notice that in the handler you only added base business logic (no deserilization,
// validation or error handler), we will add the rest with middlewares

const inputSchema = {
  type: 'object',
  properties: {
    body: {
      type: 'object',
      properties: {
        creditCardNumber: { type: 'string', minLength: 12, maxLength: 19, pattern: '\d+' },
        expiryMonth: { type: 'integer', minimum: 1, maximum: 12 },
        expiryYear: { type: 'integer', minimum: 2017, maximum: 2027 },
        cvc: { type: 'string', minLength: 3, maxLength: 4, pattern: '\d+' },
        nameOnCard: { type: 'string' },
        amount: { type: 'number' }
      required: ['creditCardNumber'] // Insert here all required event properties

// Let's "middyfy" our handler, then we will be able to attach middlewares to it
const handler = middy(processPayment)
  .use(urlEncodeBodyParser()) // parses the request body when it's a JSON and converts it to an object
  .use(validator({inputSchema})) // validates the input
  .use(httpErrorHandler()) // handles common http errors and returns proper responses

module.exports = { handler }


As simple as:

npm install middy


yarn add middy


Middy has been built to work by default from Node >= 6.10.

If you need to run it in earlier versions of Node (eg. 4.3) then you will have to transpile middy's code yourself using babel or a similar tool.


One of the main strengths of serverless and AWS Lambda is that, from a developer perspective, your focus is mostly shifted toward implementing business logic.

Anyway, when you are writing a handler, you still have to deal with some common technical concerns outside business logic, like input parsing and validation, output serialization, error handling, etc.

Very often, all this necessary code ends up polluting the pure business logic code in your handlers, making the code harder to read and to maintain.

In other contexts, like generic web frameworks (express, fastify, hapi, etc.), this problem has been solved using the middleware pattern.

This pattern allows developers to isolate these common technical concerns into "steps" that decorate the main business logic code. Middleware functions are generally written as independent modules and then plugged in into the application in a configuration step, thus not polluting the main business logic code that remains clean, readable and easy to maintain.

Since we couldn't find a similar approach for AWS Lambda handlers, we decided to create middy, our own middleware framework for serverless in AWS land.


As you might have already got from our first example here, using middy is very simple and requires just few steps:

  1. Write your Lambda handlers as usual, focusing mostly on implementing the bare business logic for them.
  2. Import middy and all the middlewares you want to use
  3. Wrap your handler in the middy() factory function. This will return a new enhanced instance of your original handler, to which you will be able to attach the middlewares you need.
  4. Attach all the middlewares you need using the function .use(somemiddleware())


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

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

const handler = middy(originalHandler)


module.exports = { handler }

You can also attach inline middlewares by using the functions .before, .after and .onError.

For a more detailed use case and examples check the Writing a middleware section and the API section.

How it works

Middy implements the classic onion-like middleware pattern, with some peculiar details.

Middy middleware engine diagram

When you attach a new middleware this will wrap the business logic contained in the handler in two separate steps.

When another middleware is attached this will wrap the handler again and it will be wrapped by all the previously added middlewares in order, creating multiple layers for interacting with the request (event) and the response.

This way the request-response cycle flows through all the middlewares, the handler and all the middlewares again, giving the opportunity within every step to modify or enrich the current request, context or the response.

Execution order

Middlewares have two phases: before and after.

The before phase, happens before the handler is executed. In this code the response is not created yet, so you will have access only to the request.

The after phase, happens after the handler is executed. In this code you will have access to both the request and the response.

If you have three middlewares attached as in the image above this is the expected order of execution:

  • middleware1 (before)
  • middleware2 (before)
  • middleware3 (before)
  • handler
  • middleware3 (after)
  • middleware2 (after)
  • middleware1 (after)

Notice that in the after phase, middlewares are executed in inverted order, this way the first handler attached is the one with the highest priority as it will be the first able to change the request and last able to modify the response before it gets sent to the user.

Interrupt middleware execution early

Some middlewares might need to stop the whole execution flow and return a response immediately.

If you want to do this you can invoke handler.callback in your middleware and return early without invoking next.

Note: this will totally stop the execution of successive middlewares in any phase (before and after) and returns an early response (or an error) directly at the Lambda level. If your middlewares do a specific task on every request like output serialization or error handling, these won't be invoked in this case.

In this example we can use this capability for building a sample caching middleware:

// some function that calculates the cache id based on the current event
const calculateCacheId = (event) => { /* ... */ }
const storage = {}

// middleware
const cacheMiddleware = (options) => {
  let cacheKey
  return ({
    before: (handler, next) => {
      cacheKey = options.calculateCacheId(handler.event)
      if ( {
        // exits early and returns the value from the cache if it's already there
        return handler.callback(null,[cacheKey])

      return next()
    after: (handler, next) => {
      // stores the calculated response in the cache[cacheKey] = handler.response

// sample usage
const handler = middy((event, context, callback) => { /* ... */ })
    calculateCacheId, storage

Handling errors

But what happens when there is an error?

When there is an error, the regular control flow is stopped and the execution is moved back to all the middlewares that implements a special phase called onError, following the order they have been attached.

Every onError middleware can decide to handle the error and create a proper response or to delegate the error to the next middleware.

When a middleware handles the error and creates a response, the execution is still propagated to all the other error middlewares and they have a chance to update or replace the response as needed. At the end of the error middlewares sequence, the response is returned to the user.

If no middleware manages the error, the Lambda execution fails reporting the unmanaged error.

Promise support

Middy allows you to return promises (or throw errors) from your handlers (instead of calling callback()) and middlewares (instead of calling next()).

Here is an example of a handler that returns a promise:

middy((event, context, callback) => {
  return someAsyncStuff()
    .then(() => {
      return someOtherAsyncStuff()
    .then(() => {
      return {foo: bar}

And here is an example of a middleware that returns a similar promise:

const asyncValidator = () => {
  before: (handler) => {
    if (handler.event.body) {
      return someAsyncStuff(handler.event.body)
        .then(() => {
          return {foo: bar}

    return Promise.resolve()


Using async/await

Node.js 8.10 supports async/await, allowing you to work with promises in a way that makes handling asynchronous logic easier to reason about and asynchronous code easier to read.

You can still use async/await if you're running AWS Lambda on Node.js 6.10, but you will need to transpile your async/await code (e.g. using babel).

Take the following code as an example of a handler written with async/await:

middy(async (event, context) => {
  await someAsyncStuff()
  await someOtherAsyncStuff()

  return ({foo: bar})

And here is an example of a middleware written with async/await:

const asyncValidator = () => {
  before: async (handler) => {
    if (handler.event.body) {
      await asyncValidate(handler.event.body)

      return {foo: bar}



Writing a middleware

A middleware is an object that should contain at least 1 of 3 possible keys:

  1. before: a function that is executed in the before phase
  2. after: a function that is executed in the after phase
  3. onError: a function that is executed in case of errors

before, after and onError functions need to have the following signature:

function (handler, next) {
  // ...


  • handler: is a reference to the current context and it allows access to (and modification of) the current event (request), the response (in the after phase) and error (in case of an error).
  • next: is a callback function that needs to be invoked when the middleware has finished its job so that the next middleware can be invoked

Configurable middlewares

In order to make middlewares configurable they are generally exported as a function that accepts a configuration object. This function should then return the middleware object with before, after and onError as keys.


# myMiddleware.js

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

With this convention in mind, using a middleware will always look like the following example:

const middy = require('middy')
const myMiddleware = require('myMiddleware')

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

  option1: 'foo',
  option2: 'bar'

module.exports = { handler }

Inline middlewares

Sometimes you want to create handlers that serve a very small need and that are not necessarily re-usable. In such cases you probably will need to hook only into one of the different phases (before, after or onError).

In these cases you can use inline middlewares which are shortcut functions to hook logic into Middy's control flow.

Let's see how inline middlewares work with a simple example:

const middy = require('middy')

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

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

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

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

module.exports = { handler }

As you can see above, a middy instance also exposes the before, after and onError methods to allow you to quickly hook-in simple inline middlewares.

More details on creating middlewares

Check the code for existing middlewares to see more examples on how to write a middleware.

Available middlewares

Currently available middlewares:

For dedicated documentation on available middlewares check out the Middlewares documentation


middy(handler) β‡’ middy

Middy factory function. Use it to wrap your existing handler to enable middlewares on it.

Kind: global function
Returns: middy - - a middy instance

Param Type Description
handler function your original AWS Lambda function

middy : function

Kind: global typedef

Param Type Description
event Object the AWS Lambda event from the original handler
context Object the AWS Lambda context from the original handler
callback function the AWS Lambda callback from the original handler


Name Type Description
use useFunction attach a new middleware
before middlewareAttachFunction attach a new before-only middleware
after middlewareAttachFunction attach a new after-only middleware
onError middlewareAttachFunction attach a new error-handler-only middleware
__middlewares Object contains the list of all the attached middlewares organised by type (before, after, onError). To be used only for testing and debugging purposes

useFunction β‡’ middy

Kind: global typedef

Type Description
middlewareObject the middleware object to attach

middlewareAttachFunction β‡’ middy

Kind: global typedef

Type Description
middlewareFunction the middleware function to attach

middlewareNextFunction : function

Kind: global typedef

Param Type Description
error error An optional error object to pass in case an error occurred

middlewareFunction β‡’ void | Promise

Kind: global typedef
Returns: void | Promise - - A middleware can return a Promise instead of using the next function as a callback. In this case middy will wait for the promise to resolve (or reject) and it will automatically propagate the result to the next middleware.

Param Type Description
handler function the original handler function. It will expose properties event, context, response, error and callback that can be used to interact with the middleware lifecycle
next middlewareNextFunction the callback to invoke to pass the control to the next middleware

middlewareObject : Object

Kind: global typedef

Name Type Description
before middlewareFunction the middleware function to attach as before middleware
after middlewareFunction the middleware function to attach as after middleware
onError middlewareFunction the middleware function to attach as error middleware


Middy exports Typescript compatible type information. To enable the use of Middy in your Typescript project please make sure tsconfig.json is configured as follows:

	"compilerOptions": {
		/* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
		"esModuleInterop": true,

After that you can import middy from 'middy'; in your http handler and use it as described above.

3rd party middlewares

Here's a collection of some 3rd party middlewares and libraries that you can use with Middy:


Everyone is very welcome to contribute to this repository. Feel free to raise issues or to submit Pull Requests.


Licensed under MIT License. Copyright (c) 2017-2018 Luciano Mammino and the Middy team.

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