I'm building a contact form in nodeJS by using Hapi.js framework. I use Joi validator to validate input from the user at the server side. My doubt is whether I should use captcha to prevent spam or not. Joi could be enough?
After reading through Joi's docs, it's pretty clear that it's just an input validation tool. Validating a user's input is not always the same as preventing an input from being spammed, though.
If you are concerned about being spammed, there are several techniques to protect against it. Using a CAPTCHA is one technique, but the tradeoff is that most users find CAPTCHAS tolerable at best. Details on that here.
One method for anti-spam that just so happens to use validation is the Honey Pot technique. However, if you use that approach, make sure you're validating on the server side and not the client side. Client side validation is only for the user's convenience (since JavaScript can be disabled/bypassed).
There are many other techniques with their own pros and cons depending on what you're protecting - too many to include here. Googling or searching SO for "anti-spam techniques" or "how to prevent spam" should give you plenty of resources for inspiration.