How to add custom validator function in Joi?

Question

I have Joi schema and want to add a custom validator for validating data which isn't possible with default Joi validators.

Currently, I'm using the version 16.1.7 of Joi

   const method = (value, helpers) => {
      // for example if the username value is (something) then it will throw an error with flowing message but it throws an error inside (value) object without error message. It should throw error inside the (error) object with a proper error message

      if (value === "something") {
        return new Error("something is not allowed as username");
      }

      // Return the value unchanged
      return value;
    };

    const createProfileSchema = Joi.object().keys({
      username: Joi.string()
        .required()
        .trim()
        .empty()
        .min(5)
        .max(20)
        .lowercase()
        .custom(method, "custom validation")
    });

    const { error,value } = createProfileSchema.validate({ username: "something" });

    console.log(value); // returns {username: Error}
    console.log(error); // returns undefined

But I couldn't implement it the right way. I read Joi documents but it seems a little bit confusing to me. Can anyone help me to figure it out?

Answer 1

Your custom method must be like this:

const method = (value, helpers) => {
  // for example if the username value is (something) then it will throw an error with flowing message but it throws an error inside (value) object without error message. It should throw error inside the (error) object with a proper error message

  if (value === "something") {
    return helpers.error("any.invalid");
  }

  // Return the value unchanged
  return value;
};

Docs:

https://github.com/hapijs/joi/blob/master/API.md#anycustommethod-description

Output for value :

{ username: 'something' }

Output for error:

[Error [ValidationError]: "username" contains an invalid value] {
  _original: { username: 'something' },
  details: [
    {
      message: '"username" contains an invalid value',
      path: [Array],
      type: 'any.invalid',
      context: [Object]
    }
  ]
}