Error responses

You can respond to the intercepted request with an error response by constructing a valid error Response instance and returning it from the response resolver. This implies using an error status code (4xx/5xx) and optionally providing any custom response payload, such as an error message.

http.get<{ id: string }>('/books/:id', ({ params }) => {
  if (params.id === 'abc-123') {
    return new HttpResponse(null, { status: 404 })
  }
 
  return HttpResponse.json({
    id: params.id,
    title: 'The Lord of the Rings',
  })
})

Type safety

Type safety of your error responses depends on how you define them. When using shorthands that infer the expected response body type, like HttpResponse.text() and HttpResponse.json(), error responses will remain type-safe:

http.get<never, never { id: string } | { error: message }>('/resource', ({ request }) => {
  if (!request.headers.has('authorization')) {
    return HttpResponse.json(
      { error: 'Unauthorized' },
      { status: 401 }
    )
  }
 
  return HttpResponse.json({
    id: 'abc-123',
  })
})

Using the plain Response instance ignores any response body types because its bodyInit type cannot be narrowed—it’s always either null, string, or ReadableStream. Use this to your advantage for error responses without a body.