How to validate string with Joi?
<p>I am using <code>Node Joi</code> for validation. I am new in node I want to validate <code>env</code> to accept only 2 words "Yes" or "No" What changes I have to make in the following code </p>

<pre><code>schema = Joi.object().keys({
    app_id: Joi.string().required(),
    env: Joi.string().required()
});
</code></pre>