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validator

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

Want to use another validator? Try one of the community validators:

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 http-content-negotiation to load localized translations for the error messages (based on the currently requested language). This feature uses internally ajv-ftl-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.

Install​

To install this middleware you can use NPM:

npm install --save @middy/validator
npm install --save-dev ajv-cmd # Optional: for pre-transpiling

Options​

  • eventSchema (function) (default undefined): The compiled ajv validator that will be used to validate the input (request.event) of the Lambda handler.
  • contextSchema (function) (default undefined): The 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 compiled ajv validator that will be used to validate the output (request.response) of the Lambda handler.
  • i18nEnabled (boolean) (default true): Option to disable i18n default package.
  • defaultLanguage (string) (default en): When language not found, what language to fallback to.
  • languages (object) (default: {}): Localization overrides

NOTES:

  • At least one of eventSchema or responseSchema is required.
  • If you'd like to have the error details as part of the response, it will need to be handled separately. You can access them from request.error.cause.data, the original response can be found at request.error.response.
  • Important Transpiling schemas & locales on the fly will cause a 50-150ms performance hit during cold start for simple JSON Schemas. Precompiling is highly recommended.

transpileSchema​

Transpile JSON-Schema in to JavaScript. Default ajv plugins used: ajv-i18n, ajv-formats, ajv-formats-draft2019, ajv-keywords, ajv-errors.

  • schema (object) (required): JSON-Schema object
  • ajvOptions (object) (default undefined): Options to pass to ajv class constructor. Defaults are { strict: true, coerceTypes: 'array', allErrors: true, useDefaults: 'empty', messages: true }.

transpileLocale​

Transpile Fluent (.ftl) localization file into ajv compatible format. Allows the overriding of the default messages and adds support for multi-language errrorMessages.

  • ftl (string) (required): Contents of an ftl file to be transpiled.

Sample usage​

Example for event validation:

import middy from '@middy/core'
import validator from '@middy/validator'
import { transpileSchema } from '@middy/validator/transpile'

const lambdaHandler = (event, context) => {
  return {}
}

const schema = {
  type: 'object',
  required: ['body', 'foo'],
  properties: {
    // this will pass validation
    body: {
      type: 'string'
    },
    // this won't as it won't be in the event
    foo: {
      type: 'string'
    }
  }
}

export const handler = middy()
  .use(
    validator({
      eventSchema: transpileSchema(schema)
    })
  )
  .handler(lambdaHandler)

// 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')
})

Example for response validation:

import middy from '@middy/core'
import validator from '@middy/validator'
import { transpileSchema } from '@middy/validator/transpile'

const lambdaHandler = (event, context) => {
  return {}
}

const responseSchema = transpileSchema({
  type: 'object',
  required: ['body', 'statusCode'],
  properties: {
    body: {
      type: 'object'
    },
    statusCode: {
      type: 'number'
    }
  }
})

export const handler = middy()
  .use(validator({ responseSchema }))
  .handler(lambdaHandler)

//
handler({}, {}, (err, response) => {
  t.not(err, null)
  t.is(err.message, 'Response object failed validation')
  expect(response).not.toBe(null)
  // it doesn't destroy the response so it can be used by other middlewares
})

Example for body validation:

import middy from '@middy/core'
import httpJsonBodyParser from '@middy/http-json-body-parser'
import validator from '@middy/validator'
import { transpileSchema } from '@middy/validator/transpile'

const lambdaHandler = (event, context) => {
  return {}
}

const eventSchema = {
  type: 'object',
  required: ['body'],
  properties: {
    body: {
      type: 'object',
      required: ['name', 'email'],
      properties: {
        name: { type: 'string' },
        email: { type: 'string', format: 'email' }
        // schema options https://ajv.js.org/json-schema.html#json-data-type
      }
    }
  }
}

export const handler = middy()
  // to validate the body we need to parse it first
  .use(httpJsonBodyParser())
  .use(
    validator({
      eventSchema: transpileSchema(eventSchema)
    })
  )
  .handler(lambdaHandler)

Run a build script to before running tests & deployment.

#!/usr/bin/env bash

# This is an example, should be customize to meet ones needs
# Powered by `ajv-cmd`
# $ ajv --help

bundle () {
  ajv validate ${1} --valid \
    --strict true --coerce-types array --all-errors true --use-defaults empty
  ajv transpile ${1} \
  --strict true --coerce-types array --all-errors true --use-defaults empty \
  -o ${1%.json}.js
}

for file in handlers/*/schema.*.json; do
  bundle $file
done

locale () {
  LOCALE=$(basename ${1%.ftl})
  ajv ftl ${1} --locale ${LOCALE} -o ${1%.ftl}.js
}

for file in handlers/*/*.ftl; do
  locale $file
done
import middy from '@middy/core'
import validator from '@middy/validator'
import eventSchema from './schema.event.js'
import en from './en.js'
import fr from './fr.js'

const lambdaHandler = (event, context) => {
  return {}
}

export const handler = middy()
  .use(
    validator({
      eventSchema,
      languages: { en, fr }
    })
  )
  .handler(lambdaHandler)

Transpile during cold-start​

import { readFile } from 'node:fs/promises'
import middy from '@middy/core'
import validator from '@middy/validator'
import { transpileSchema, transpileLocale } from '@middy/validator/transpile'
import eventSchema from './schema.event.json'

const lambdaHandler = (event, context) => {
  return {}
}

const en = transpileLocale(await readFile('./en.ftl'))
const fr = transpileLocale(await readFile('./fr.ftl'))

export const handler = middy()
  .use(
    validator({
      eventSchema: transpileSchema(eventSchema),
      languages: { en, fr }
    })
  )
  .handler(lambdaHandler)

Transpile during cold-start with default messages​

import { readFile } from 'node:fs/promises'
import middy from '@middy/core'
import validator from '@middy/validator'
import { transpileSchema, transpileLocale } from '@middy/validator/transpile'
import { en, fr } from 'ajv-ftl-i18n' // `ajv-i18n` can also be used
import eventSchema from './schema.event.json'

const lambdaHandler = (event, context) => {
  return {}
}

export const handler = middy()
  .use(
    validator({
      eventSchema: transpileSchema(eventSchema),
      languages: { en, fr }
    })
  )
  .handler(lambdaHandler)