# YAGNI Principle

A design principle from Extreme Programming (XP) states that **a programmer shouldn't add any functionality until it's actually necessary.**

Ron Jeffries, one of the founders of XP wrote that programmers should "**always implement things when you actually need them, never when you foresee that you need them**" and "do the simplest possible thing that will work".

 **Why is it useful?**

* This can help you iterate really quickly.
* It helps prevent design fatigue or fear of a large all-encompassing up-front design.

**But!**

YAGNI is meant to be combined with **continuous refactoring**, automated unit testing, and continuous integration.

Failure to refactor code early could require huge amounts of **technical debt** and **rework**.

Therefore, in order to do YAGNI well, developers first need to know how to:

* write code that's testable
* run automated tests