oneOf schemas not matching when an object has a custom toJSON() method

[unek]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.15.0

Plugin version

No response

Node.js version

18.12.1

Operating system

Linux

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

Ubuntu 22.10

Description

toJSON() is not called on objects before attempting to match a schema in oneOf, resulting in an error "does not match schema definition". When oneOf is not used, the same schema matches, works, and serializes.

Steps to Reproduce

const fastJson = require('fast-json-stringify');

// schema using oneOf
const stringifyOneOf = fastJson({
  description: 'transfer failed.',
  oneOf: [
    {
      type: 'object',
      description: 'Invalid withdrawal address provided.',
      properties: {
        error: { type: 'string', const: 'INVALID_ADDRESS' },
      },
      required: ['error'],
      additionalProperties: false,
    },
    {
      type: 'object',
      description: 'Cannot afford this transfer + fee.',
      properties: {
        error: { type: 'string', const: 'INSUFFICIENT_ACCOUNT_BALANCE' },
        balance: {
          type: 'object',
          description: 'Available balance for this currency.',
          properties: {
            value: { type: 'string' },
            currency: { type: 'string' },
          },
          additionalProperties: false,
          required: ['value', 'currency'],
        },
      },
      required: ['error', 'balance'],
      additionalProperties: false,
    },
  ],
});

// schema with no oneOf
const stringify = fastJson({
  type: 'object',
  description: 'Cannot afford this transfer + fee.',
  properties: {
    error: { type: 'string', const: 'INSUFFICIENT_ACCOUNT_BALANCE' },
    balance: {
      type: 'object',
      description: 'Available balance for this currency.',
      properties: {
        value: { type: 'string' },
        currency: { type: 'string' },
      },
      additionalProperties: false,
      required: ['value', 'currency'],
    },
  },
  required: ['error', 'balance'],
  additionalProperties: false,
});

// custom class with toJSON() method
class Balance {
  constructor(value, currency) {
    this._value = value;
    this._currency = currency;
  }

  toJSON() {
    return {
      value: this._value,
      currency: this._currency,
    };
  }
}

const balance = new Balance(100, 'USD');

// will throw 'The value of '#' does not match schema definition.'
console.log(stringifyOneOf({
  error: 'INSUFFICIENT_ACCOUNT_BALANCE',
  balance,
}));

// stringifies as expected
console.log(stringify({
  error: 'INSUFFICIENT_ACCOUNT_BALANCE',
  balance,
}));

Expected Behavior

the schema inside oneOf should match as it is the same schema that works when used outside of oneOf - toJSON() not called on objects before matching?

[ivan-tymoshenko]

@unek Hi, when you add the oneOf, anyOf or if/then keywords to the schema, FJS validates your input data to match it with one of the options. It validates it before the toJSONcall happens, so it seems like an expected behavior for me.