Validate the Fastify input with Joi

If you are an hapi developer, you may know the joi library. It is a powerful validation library that allows you to define programmatic schemas for your data and validate them with ease.

One of the hardest things about migrating from hapi to fastify is the complexity that’s involved for you to be able to keep using joi for the route's input validation.

But now, thanks to the joi-compiler library, you can use joi with fastify without any problem!

Note If you have been afraid to use it because it has very few downloads, don't worry — it is a new library and I'm the author. And, if you don't know me, I'm a Fastify core maintainer and I'm the co-author of the Fastify book too!

The joi-compiler module replaces the default ajv schema compiler with a joi-based one and allows you to use joi schemas to validate your data. Let's see how to use it!

Integrate joi with fastify

After you have installed the latest fastify@4 and joi-compiler module:

mkdir fastify-joi
cd fastify-joi
npm init -y
npm install --save fastify@4 joi-compiler

We can create a simple index.js file with the following content:

const fastify = require('fastify')
const JoiCompiler = require('joi-compiler')

// Instantiate the compiler
const joiCompilerInstance = JoiCompiler()

// Install it to Fastify
const app = fastify({
  schemaController: {
    bucket: joiCompilerInstance.bucket,
    compilersFactory: {
      buildValidator: joiCompilerInstance.buildValidator

// Use it!
const joiSchema = joiCompilerInstance.joi.object({
  foo: joiCompilerInstance.joi.string().required()

app.get('/', {
  handler: () => 'Hello World!',
  schema: {
    headers: joiSchema

app.listen({ port: 3000 })

If you run this code, you will be able to test the joi validation with the following command:

curl -X GET http://localhost:3000/
# {"statusCode":400,"error":"Bad Request","message":"\"foo\" is required"}

curl -X GET http://localhost:3000/ -H "foo: bar"
# Hello World!

As you can see, the joi schema is working as expected! In a few lines of code, we have integrated joi with fastify and we are able to use it to validate the input of your application routes.

How it works

The joi-compiler module is a Fastify schema compiler, it is used by Fastify to build the components during the startup to guarantee the encapsulated feature.

For this reason, the joi-compiler can be configured in the schemaController option during the Fastify application creation.

How the Fastify Schema Controller works is out of the scope of this article. But if you want to know more about it, you can read the Fastify book!

How to configure the joi-compiler

The JoiCompiler accepts an optional configuration object to customize the joi instance.

The default configuration is:

const joiCompilerInstance = JoiCompiler({
  // optionally: provide all the JOI options you need
  // Here is all the possible options:
  prefs: {
    stripUnknown: true

  // optionally: an array with custom JOI extensions such as `@joi/date`
  extensions: [

  // optionally: if you want to use the async validation. Default: false
  asyncValidation: false

When you instantiate the joiCompilerInstance, the returned object has the following properties:

  • buildValidator: The function to pass to the schemaController option of Fastify.

  • bucket: The joi bucket that contains the schemas when you call the app.addSchema(joiSchema) method. You can omit this option if you don't use app.addSchema.

  • joi: A customized joi instance that contains the installed extensions, if there are any. It is a good practice to use this instance to build your schemas.

How to use ajv and joi together!

The power of Fastify is that you can use different schema compilers at the same time! Here is an example of how you can learn from the Fastify book:

const joiCompilerInstance = JoiCompiler()
const app = fastify()'/ajv', {
  handler: (request) => request.body,
  schema: {
    body: {
      type: 'object',
      properties: {
        toX: { const: 42 },
        toY: { const: 50 }

app.register(async function pluginJoi (app, opts) {
  // Install the joi compiler into this encapsulated context!
    bucket: joiCompilerInstance.bucket,
    compilersFactory: {
      buildValidator: joiCompilerInstance.buildValidator

  // Let's try to use the external schemas!
  app.addSchema({ $id: 'x', $value: 42 })
  app.addSchema({ $id: 'y', $value: 50 })'/joi', {
    handler: (request) => request.body,
    schema: {
      body: Joi.object({
        toX: Joi.ref('$x'),
        toY: Joi.ref('$y')

app.listen({ port: 3000 })

If you run this code, you will be able to use the ajv and joi validation in the same application! The ajv validation will be used for the /ajv route and the joi validation will be used for the /joi route.

Pretty cool, right?


The joi-compiler is a powerful tool that allows you to build and manage joi instances for Fastify out of the box in a few lines of code. If you're a joi user, joi-compiler is definitely worth checking out!

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