3.0.0 • Public • Published

Middy AJV

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AJV middleware for the middy framework, the stylish Node.js middleware engine for AWS Lambda

This middleware automatically validates incoming events and outgoing responses against custom schemas defined with the JSON schema syntax.

If an incoming event fails validation a BadRequest error is raised. If an outgoing response fails validation a InternalServerError error is raised.

This middleware can be used in combination with httpErrorHandler to automatically return the right response to the user.

It can also be used in combination with httpcontentnegotiation to load localised translations for the error messages (based on the currently requested language). This feature uses internally ajv-i18n module, so reference to this module for options and more advanced use cases. By default the language used will be English (en), but you can redefine the default language by passing it in the ajvOptions options with the key defaultLanguage and specifying as value one of the supported locales.

Also, this middleware accepts an object with plugins to be applied to customize the internal ajv instance.


To install this middleware you can use NPM:

npm install --save middy-ajv

Requires: @middy/core:>=2.0.0


  • eventSchema (function) (default undefined): The JSON schema compiled ajv validator that will be used to validate the input (request.event) of the Lambda handler.
  • contextSchema (function) (default undefined): The JSON schema object or compiled ajv validator that will be used to validate the input (request.context) of the Lambda handler. Has additional support for typeof keyword to allow validation of "typeof":"function".
  • responseSchema (function) (default undefined): The JSON schema compiled ajv validator that will be used to validate the output (request.response) of the Lambda handler.
  • availableLanguages (object) (optional): Error messages can be returned in multiple languages using ajv-i18n. Language is selected based on event.preferredLanguage set by @middy/http-content-negotiation. Should be in the format: { 'en': require('ajv-i18n/localize/en') }.
  • defaultLanguage (string) (default: en): The default language to use when availableLanguages is provided and event.preferredLanguage is not supported.


  • At least one of inputSchema or outputSchema is required.

Sample usage

Example for validation using precompiled schema:

import middy from '@middy/core'
import validator from 'middy-ajv'

const handler = middy((event, context) => {
  return {}

const eventSchema = require('schema.js')

handler.use(validator({ eventSchema }))

// invokes the handler, note that property foo is missing
const event = {
  body: JSON.stringify({something: 'somethingelse'})
handler(event, {}, (err, res) => {
  t.is(err.message,'Event object failed validation')

Build step

Folder Structure

|-- handlers
| |-- {enpoint}
| | |-- index.js
| | |-- eventSchema.json

After the build script has been run on the endpoint folder, it will contain schema.js and index.js.


$ npm install -D ajv-cli


#!/usr/bin/env bash
# Compile JSON Schemas
function ajv {
	node ./node_modules/ajv-cli/dist/index.js compile -c ajv-formats -c ajv-formats-draft2019 --strict=true --coerce-types=array --all-errors=true --use-defaults=empty --messages=false -s $1'eventSchema.json' -o $1'eventSchema.js'
for dir in handlers/*/; do
  if [ ! -n "$(ajv $dir | grep ' is valid')" ]; then
	  exit 1

Middy documentation and examples

For more documentation and examples, refers to the main Middy monorepo on GitHub or Middy official website.


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) 2017-2021 will Farrell and the Middy team.

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