JOI validation for a string with json as a value

i am trying to validate the string using the JOI package available in npm, i checked this documentation which has many useful string formats e.g. date, IP, base64 but i need to validate the following JSON which contains a stringified JSON as a value, and there is no example in the documentation for such case

{
    "id": 232,
    "name": "Trojan Horse",
    "file": "download.exe",
    "infected": true, 
    "engines": "['Norton', 'AVG', 'NOD32']"
}

So for example what if i want to check engines have valid JSON value and have at-least one engine defined if infected key is set to true

The following schema works only if the engines value is written as parsed JSON

Joi.object().keys({
    id: Joi.number().required(),
    name: Joi.string().min(5).required(),
    file: Joi.string().min(3).required(),
    infected: Joi.boolean().required(),
    engines: Joi.array().when('infected', {
        is: Joi.exists().valid(true),
        then: Joi.min(1).required()
    })
});

Answer

What you need to do is to create a custom JOI validator by extending the array validator of the JOI package and using that custom validator for the engines property.

const custom = Joi.extend({
type: 'array',
base: Joi.array(),
coerce: {
      from: 'string',
      method(value, helpers) {

          if (typeof value !== 'string' ||
              value[0] !== '[' && !/^s*[/.test(value)) {

              return;
          }

          try {
            return { value: JSON.parse(value) };
          }
          catch (ignoreErr) { }
      }
  }
});

const schema = Joi.object({
  id: Joi.number().required(),
  name: Joi.string().min(5).required(),
  file: Joi.string().min(3).required(),
  infected: Joi.boolean().required(),
  engines: custom.array().when('infected', {
      is: true,
      then: custom.array().min(1).required()
  })
})

const validateTest = async (joiSchema,  testObject) => {
  try {
    const value = await joiSchema.validateAsync(testObject);
    console.log(value);
}
catch (err) { 
  console.error(err)
 }
};

validateTest(schema, {
  "id": 232,
  "name": "Trojan Horse",
  "file": "download.exe",
  "infected": true, 
  "engines": `["Norton", "AVG", "NOD32"]`
})

You can see more examples like that here