How to customize an errors with Fastify, Ajv and Schema?

Question

How can I customize an errors with Fastify, Ajv and Scheme? I need to format by:

{
    "message": "Bad request",
    "path": "/account/register",
    "status": 400,
    "timestamp": 1697094987,
    "errors": [
      { "key": "email", "value": "This value is not a valid email address."
    ]
}

When I try to:

const routes = async (app: Application) => {
  app.post('/accounts/register', { schema }, async (req: FastifyRequest, reply: FastifyReply) => {
    reply.code(200);
  });
};
   

I get an error with the following format:

{
    "statusCode": 400,
    "code": "FST_ERR_VALIDATION",
    "error": "Bad Request",
    "message": "body must have required property 'email'"
}

Answer 1

If you need you can change the response error's message by setting the schemaErrorFormatter.

Nevertheless, you will not change the error output format.

Do change the output format you must:

• enable the AJV allErrors config. Note that it is considered insecure
• customize the Fastify error handler

const app = require('fastify')({
  logger: true,
  ajv: {
    customOptions: {
      allErrors: true
    }
  }
})

app.setErrorHandler(function (error, request, reply) {
  if (error.validation) {
    return reply.status(400).send({
      message: 'Bad request',
      path: request.url,
      status: 400,
      timestamp: Date.now(),
      errors: error.validation.map(err => ({
        key: err.params?.missingProperty || err.dataPath,
        value: err.message
      }))
    })
  }

  reply.status(500).send(error)
})

const schema = {
  body: {
    type: 'object',
    required: ['name', 'age'],
    properties: {
      name: { type: 'string' },
      age: { type: 'number' }
    }
  }
}

app.post('/', { schema }, async (request, reply) => {
  return request.body
})

app.listen({ port: 8080 })

Will produce this output:

{
  "message": "Bad request",
  "path": "/",
  "status": 400,
  "timestamp": 1697372490243,
  "errors": [
    {
      "key": "name",
      "value": "must have required property 'name'"
    },
    {
      "key": "age",
      "value": "must have required property 'age'"
    }
  ]
}