feat: drop nullable const support

[ivan-tymoshenko]

❗BREAKING CHANGES

I didn't check it here #511 but turns out if the schema has const property, it can't be nullable.

If I have a schema like that:

{ "type": ["string", "null"], "const": "abc" }

or

{ "type": "string", "nullable": true, "const": "abc" }

it will validate the null value as false. That means if we serialize the null value as null, the serialized object would fail the validation.

P.S. I was confused because I saw the validation check, but it turns out that is-my-json-valid doesn't handle const check at all.

fast-json-stringify/test/const.test.js

Line 242 in ab900c8

t.ok(validate(JSON.parse(output)), 'valid schema')

[Uzlopak]

JSON.stringify({foo: null }) is {"foo":null}

So how can it be that stringifying { foo: null } results in {"foo": "baz}

[ivan-tymoshenko]

FJS output should be equal to JSON.stringify only when you pass data that match the data schema. You shouldn't pass null as a data input for schema { "type": ["string", "null"], "const": "abc" }, because null doesn't match the schema.

[ivan-tymoshenko]

We want this to work:

const schema = { /*some json schema draft 7*/ }
const data = { /*data that match to schema or can be coerced according to FJS rules */ }

const json = fjs(data)
const parsedData = JSON.parse(json)

validate(schema, data) // should be always true

And if we serialize null as null,

validate({ "type": ["string", "null"], "const": "abc" }, null)

will return false.

[ivan-tymoshenko]

For the same reason, if you pass null to the serializer with schema { "type": "number" }, you will get 0, instead of null.

[mcollina]

Unfortunately I think we should land this in a next branch and wait for the Fastify v5 release train (2023).