How to add custom content type parser for Nest with Fastify

Question

I need to overwrite the content parser for application/json so my application accepts empty body. Right now it throws:

{
    "statusCode": 400,
    "code": "FST_ERR_CTP_EMPTY_JSON_BODY",
    "error": "Bad Request",
    "message": "Body cannot be empty when content-type is set to 'application/json'"
}

I'm trying with:

import { FastifyAdapter, NestFastifyApplication } from '@nestjs/platform-fastify';
import { NestFactory } from '@nestjs/core';

const fastifyAdapter = new FastifyAdapter({
   addContentTypeParser: // what should go here
});

const app = await NestFactory.create<NestFastifyApplication>(AppModule, fastifyAdapter);

but I can't figure out what is expected under addContentTypeParser

Answer 1

To allow empty json body, you can add a content body parser like the following. Instead of throwing FST_ERR_CTP_EMPTY_JSON_BODY error, this will set request body to null.

const fastifyAdapter = new FastifyAdapter();

fastifyAdapter.getInstance().addContentTypeParser(
  'application/json',
  { bodyLimit: 0 },
  (_request, _payload, done) => {
    done(null, null);
  }
);

You can also set the request body to any value you want using the second argument of the done method.

Setting body to an empty object for example, would be look like this:

const fastifyAdapter = new FastifyAdapter();

fastifyAdapter.getInstance().addContentTypeParser(
  'application/json',
  { bodyLimit: 0 },
  (_request, _payload, done) => {
    done(null, {});
  }
);

---

Also, for those who are getting FST_ERR_CTP_INVALID_MEDIA_TYPE error like me, adding a catch-all content type parser for empty body requests fixes the issue.

const fastifyAdapter = new FastifyAdapter();

fastifyAdapter.getInstance()
  .addContentTypeParser(
    '*',
    { bodyLimit: 0 },
    (_request, _payload, done) => {
      done(null, null);
    }
  );

Tus clients, by default sends a POST request with no content-type and an empty body. Using a catch-all parser fixed my problem.

Answer 2

I was getting the error FST_ERR_CTP_INVALID_MEDIA_TYPE because I was getting a PUT request with application/json in Content-Type header with empty body for a particular url.

I fixed the issue with a custom hook that fixes the request before passing it to the adapter. I don't know if your solution is better, but this looks cleaner to me:

const adapter = nestApp.getHttpAdapter();
const instance = adapter.getInstance();
instance.addHook('onRequest', (request, _reply, done) => {
    const contentType = request.headers['content-type'];
    const method = request.raw.method;
    const url = request.url;

    if (
        method === 'PUT' &&
        contentType === 'application/json; charset=utf-8' &&
        url.startsWith('/example')
    ) {
        delete request.headers['content-type'];
    }

    done();
});

I was inspired by this question: Use default JSON parser in Fastify addContentTypeParser

Answer 3

Because request.body will still null in onRequest and preParsing. Check body from header content-length

const fastify = Fastify({
  // logger: true,
});

fastify.addHook('preParsing', (request, reply, payload, done) => {
    if (
      ['POST', 'DELETE'].includes(request.method) &&
      request.headers['content-type'] == 'application/json' &&
      request.headers['content-length'] == '0'
    ) {
      delete request.headers['content-type'];
  }

  done();
});

const fastifyAdapter = new FastifyAdapter(fastify);
const app = await NestFactory.create<NestFastifyApplication>(AppModule, fastifyAdapter);