YAGNI principle ("You Aren't Gonna Need It") is a practice in software development which states that features should only be added when required. As a part of the extreme programming (XP) philosophy, YAGNI trims away excess and inefficiency in development to facilitate the desired increased frequency of releases.
The principle helps developers avoid wasted effort on features that are assumed to be needed at some point. The idea is that this assumption often ends up being incorrect. Even if a feature ends up being desired, it still may turn out that the implementation is not necessary. The argument is for developers to not waste time on creating extraneous elements that may not be necessary and can hinder or slow the development process.
As YAGNI helps avoid spending time on features that may not be used, the main features of a program are better developed and less total time is spent on each release. Much like the Worse is Better principle, YAGNI is against the development of extra features and helps avoid feature creep. Other popular programming principles include KISS (Keep It Simple, Stupid) and DRY (Don’t Repeat Yourself).
YAGNI is one of the best known principles of XP. This part of XP comes from the book Extreme Programing Installed, authored by Ronald Jeffries, Ann Anderson and Chet Henderson.