When `asString` receives `undefined` an exception is thrown

Prerequisites

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I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.24.3

Plugin version

5.9.1

Node.js version

20.2.0

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

Ubuntu 22.04.1

Description

Not entirely sure this is a bug, could be intended behavior.

I have an array of stings, however one of the elements is undefined.

['test1', 'test2', 'test3', undefined]

If I return that from a fastify endpoint, asString throws an exception:

TypeError: Cannot read properties of undefined (reading 'toString')
    at Serializer.asString (/repo/node_modules/fast-json-stringify/lib/serializer.js:113:19)
    at anonymous1 (eval at build (/repo/node_modules/fast-json-stringify/index.js:190:23), <anonymous>:17:28)
    at anonymous0 (eval at build (/repo/node_modules/fast-json-stringify/index.js:190:23), <anonymous>:52:19)
    at serialize (/repo/node_modules/fastify/lib/reply.js:896:12)
    at preSerializationHookEnd (/repo/node_modules/fastify/lib/reply.js:516:17)
    at preSerializationHook (/repo/node_modules/fastify/lib/reply.js:500:5)
    at Reply.send (/repo/node_modules/fastify/lib/reply.js:204:7)
    at /repo/node_modules/fastify/lib/wrapThenable.js:25:15

Possibly related:
#218
#84

Steps to Reproduce

export default async (app: FastifyInstance) => {
  app.route({
    method: 'GET',
    url: '/test',
    schema: {
      response: {
        200: {
          type: 'object',
          properties: {
            status: { type: 'string' },
            message: { type: 'string' },
            testArray: {
              type: 'array',
              items: { type: 'string' }
            }
          }
        }
      }
    },
    handler: async () => {
      return {
        status: 'ok',
        message: 'This is a test route',
        testArray: ['test1', 'test2', 'test3', undefined]
      }
    }
  })
}

Expected Behavior

It seems that asString will default the input to an empty string if its null.

fast-json-stringify/lib/serializer.js

Line 103 in 74c35c0

if (str === null) {

I can confirm this behavior by sending ['test1', 'test2', 'test3', null] instead. No error is thrown, and my response looks like the following:

{
    "status": "ok",
    "message": "This is a test route",
    "testArray": [
        "test1",
        "test2",
        "test3",
        ""
    ]
}

I'm thinking that asString should check for undefined as well. However I understand that this may technically be incorrect because my response schema defines an array of strings, not Array<string | undefined>.

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

Absolutely! I've opened pr #681. Not sure if I did that correctly sorry in advanced