How to add custom validator function in Joi?

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]
    }
  ]
}

const Joi = require('@hapi/joi');

Joi.object({
    password: Joi
        .string()
        .custom((value, helper) => {

            if (value.length < 8) {
                return helper.message("Password must be at least 8 characters long")

            } else {
                return true
            }

        })

}).validate({
    password: '1234'
});