title: Custom string validation with Joi.js
body: <p>I have an input in my API and I need to validate it and see if it is one the two string that I desire. Is there way to validate the string with Joi.js</p>
  <pre><code>type DummyValidatorInput = {
    firstInput: string;
    secondInput: string;
  }
  
  export const DummyValidator = (
    input: DummyValidatorInput,
  ): Joi.ValidationResult =&gt; {
    return Joi.object({
      firstInput: Joi.string().min(5).max(64).required(),
      secondInput: Joi.string().required()
    }).validate(input);
  };
  </code></pre>
  <p>I want to validate that if the <code>secondInput</code> is <code>customerTypeOne</code> string or <code>customerTypeTwo</code> string, is this possible with Joi?
  Any idea or suggestion is appreciated :)</p>