oneOf schemas not matching when an object has a custom toJSON() method #621

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,
}));