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JavaScript is NOT a functional language

This humorous tweet garnered a plenty of attention early last week, and I confess I contributed to the virus with my own retweet. While I’m inclined to concur with the sentiment, I’m not completely comfortable doing so. To chose a side in the discussion implies not only that functional programming is a well-defined paradigm, but also that it is a Boolean property of languages. I’m not so sure we’ve all agreed what functional programming is, nor can we say that a language wholly is or isn’t functional.

My stance is the statement is neither true nor false, at least in the way it was meant to be interpreted. I certainly agree that JavaScript is more functional than, say, Java (especially prior to version 8), but it is also much less functional than the golden standard Haskell. Notice that I am making comparisons of the languages relative to one another, rather than partitioning the languages into opposing camps. I personally believe this is the best way to view a language’s place in functional programming as a position along continuum with imperative on one end and functional on the other.

On one end we have languages which we all agree aren’t functional, such as Small Talk or Pascal. The other end of the spectrum features the heralded functional language greats Lisp, Erlang, Haskell, ML, etc. What I find interesting is what lies in the space between those two. Some languages have properties which are certainly characteristic of the functional languages, but the language itself isn’t typically regarded as functional. Languages like Java are adopting features of functional languages making itself more functional. At what point does a language cross this mystical threshold?

Not only do I believe this should be regarded as a continuum, I believe there is a better question to ask when judging where the language lies: How much does this language help me write in a functional programming style? Let’s face it, Java will fight you to the death if you want to be functional, but the advent of lambdas and powerful libraries like Google’s Guava allow you to be quite a bit more functional than classic OOP Java.

What properties help us write in a functional style? A few that immediately come to my mind are (1) treating functions as first-class citizens, (2) declarative constructs, (3) bias towards immutable data, and (4) control over side-effects. This is by no means meant to be exhaustive, but properties like these ultimately combine to allow programs to achieve referential transparency, which is arguably the most important property of a functional program. This allows the developer(s) to reason about the program. The more a language favors these patterns as the idiomatic approaches to programming, the further towards functional programming end of the spectrum we should regard it.

Back to JavaScript, which is the title of this post. It does very well in the first property I feel we need, which is treating functions as first class citizens. However, it fails miserably on the next three. Many languages like Java at least give us a final key word, but everything is mutable in JavaScript. That’s not to say that you cannot write a program which does not mutate the data. It just means the language is not on your side to help you accomplish that goal. Hence, it makes sense to write an outstanding book like Michael Fogus did with Functional JavaScript. Yet the reader should be aware that it will take much discipline to retain the ultimate goal of referential transparency.

Another thing to notice is how some languages need libraries to allow the developer to write in a more functional style. I already mentioned Guava for Java. The Functional JavaScript book utilizes underscore.js. This shows me that sometimes a language needs a little help to be functional, but it can be done.

Perhaps also of interest is how much a language will allow you to NOT be functional. For instance, Scala is widely regarded as a functional language. Yet you can write some very filthy imperative code thanks to var declarations and easy-to-reach side-effecting calls such as println. Compare that to Haskell which requires you to rope off the entire function with caution tape like a crime scene to allow an I/O operation to occur. Hence, we could use this to say that Haskell helps developers write more functional code than Scala.

So I guess I agree that JavaScript isn’t a functional language, but only in the spirit that I don’t think it makes a lot of sense to declare languages as such, one way or the other. Rather we should make statements like “JavaScript helps me write in a functional style more than Java, but not as much as CoffeeScript which allows me write with more declarative expressions.” Even my most recent post has me regarding my ideal language as being functional. However, I think from now on I’m going to adopt the verbiage of stating how much I believe a language helps me write functional-style programs. I suspect this mentality will be more useful for discussing programming languages. It strikes me as being a little more objective therefore less likely to spark passionate-yet-pointless arguments about language design.

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Olde Comments
  1. Arguably both functional and object-oriented programming can be done in any programming language. Some languages have better facilities for some or both styles.
    Object-oriented programming would even be possible in Haskell although I guess it’ll be a bit weird to do.

    • barnesjd says:

      That’s right. I think that’s why I like this new perspective on languages in terms of OOP and FP. As someone noted on twitter, FP is a style not a feature of the language.

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