One of the most important changes in v2 is the new API design. We carefully refined every function to make date-fns consistent, predictable and reliable. In this post, I tell about goals and values that helped us to design simple API that is pleasant to use.
date-fns v2 introduces many new features and breaking changes and this post is a part of a series #date_fns_v2 where we describe in detail most notable ones.
When we just started working on date-fns, our only goal was to build a library for working with dates in functional style. For years we were adding more and more functions, often copying and adapting Moment.js API without second look on it.
After a while, we realized that our API and behavior is not as consistent as we want them to be. Arguments in functions doing a similar job were not always in the same order and had a different naming scheme. Sometimes functions were throwing exceptions where they shouldn't and weren't throwing when they should. I'm not talking about coercion logic; it was behind even our understanding.
The need for changes was obvious, but first, we had to define goals and values, that would help us make decisions in ambiguous cases.
Stick to existing standards. Instead of reinventing the wheel and relying on our subjective opinion we decided to look for current standards and well-established practices. That will provide the best compatibility with other languages and save us from mistakes that others made years if not decades ago.
Consistency. We want date-fns to be as predictable and easy to understand as possible. Function names, the order of arguments and the behavior must be consistent across the whole library.
Explicitness. date-fns should prefer explicitness over implicitness. Sometimes the latter helps to make code look cleaner. You know that feeling when a library does what you wanted it to do without saying a word. But more often it causes bugs that are hard to debug or even worse to notice.
Convenience. We want date-fns to be pleasant to use. We should help developers to avoid mistakes.