Error codes

The plugin throws two FastifyError subclasses, both with statusCode: 400. They are created with @fastify/error, so each has a stable code you can match against, plus a statusCode of 400.

Reference

Constructor	code	statusCode	When
MissingContentTypeError	FST_ERR_QUERY_MISSING_CONTENT_TYPE	400	QUERY request arrived without a Content-Type header.
EmptyBodyError	FST_ERR_QUERY_EMPTY_BODY	400	QUERY request arrived with no body (no Content-Length > 0 and no Transfer-Encoding).

Source

Both constructors are created in index.js and re-exported from types/index.d.ts.

// index.js
export const MissingContentTypeError = createError(
  'FST_ERR_QUERY_MISSING_CONTENT_TYPE',
  'QUERY requests must include a Content-Type header',
  400
)

export const EmptyBodyError = createError(
  'FST_ERR_QUERY_EMPTY_BODY',
  'QUERY requests must include a request body',
  400
)

Matching them

The exported constructors are real classes, so you can use instanceof:

import {
  MissingContentTypeError,
  EmptyBodyError
} from '@thecodepace/fastify-http-query'

app.setErrorHandler((error, request, reply) => {
  if (error instanceof MissingContentTypeError) {
    // Surface a friendlier message; status is already 400.
    reply.send({ error: 'missing_content_type' })
    return
  }
  if (error instanceof EmptyBodyError) {
    reply.send({ error: 'empty_body' })
    return
  }
  reply.send(error)
})

Or match on the code string if you would rather not import the classes:

app.setErrorHandler((error, request, reply) => {
  if (error.code === 'FST_ERR_QUERY_MISSING_CONTENT_TYPE') { /* ... */ }
  if (error.code === 'FST_ERR_QUERY_EMPTY_BODY') { /* ... */ }
  reply.send(error)
})

Why these errors exist

Both errors enforce the parts of draft-ietf-httpbis-safe-method-w-body that a server is required to enforce:

• "Servers MUST fail the request if the Content-Type request field is missing or is inconsistent with the request content."
• "The content of the request and its media type define the query." A QUERY request without content has no query to perform.