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I come from a background in Haskell. I'm very used to getting things done with recursive functions and the typical higher-order functions (folds, maps, filters, etc) and composing functions together. I'm developing in node.js now, and I'm seriously tempted to write my own modules implementing these functions so I can use them in my code in a way that makes sense to me.

My question is, basically: is Javascript set up to handle this type of burden? I understand that the aforementioned recursive functions can be easily refactored into iterative ones, but often times I find myself calling a lot of functions within functions, and I don't know if Javascript can handle this type of thing well. I know that things like Underscore exist and implement some FP principles, but my question basically boils down to: is it good practice to program functionally in Javascript? If not, why not?

I also apologize if this question is a little too soft for SO, but I don't want to start putting together my own tool set if it's just going to break everything once it gets too large.

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  • Probably depends a bit. JavaScript isn't tail-call optimized, so I'd imagine that'll be an issue at some point. I can't say from experience, but I'm guessing that there's generally, or at least traditionally, more overhead WRT function calls in JS than in Haskell. Things may change a bit for the better in the next version of JS. I believe its new function syntax will allow for TCO. Commented Mar 29, 2014 at 16:43
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    This blog post may interest you, though not directly related to the question. Commented Mar 29, 2014 at 16:47
  • I don't have an answer for you, but I do have an anecdote: it was actually Javascript that sold me on FP. I used to be an OO programmer whose main "serious" language was Java. While working on a web-based calendar application, I discovered I could write Javascript functions that accepted Javascript functions as input and spat out Javascript functions as output. The power and compactness was enthralling; I've moved on to more serious FP languages, but I still remember that experience fondly. Commented Apr 6, 2014 at 17:58

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In my opinion, the short answer to your question is yes -- applying functional programming principles is viable in Javascript! (I believe that this is also true for most other languages -- there's usually something to be gained from applying FP principles).

Here's an example of a functional parser combinator library I built in Javascript. (And here it is in action). It was important to be functional because: 1) it allows me to build parsers by composition, which means I can build and test small parsers independently, then put them together and have confidence that the behavior will be the same, and 2) it makes backtracking super easy to get right (which is important because the choice operator backtracks when an alternative fails).

So these are FP principles (note the absence of recursion, folds, maps, and filters from this list) that I found extremely useful in building my library:

  • avoiding mutable state
  • pure functions (i.e. output depends only on input)
  • composition: building complex apps by gluing together simple pieces

It's usually quite easy and pleasant to apply these in Javascript because of Javascript's support for:

  • first-class functions
  • anonymous functions
  • lexical closures

but here are some things to watch out for:

  • lack of popular library of efficient functional data structures
  • lack of tail-call optimization (at least at the moment)
  • partial application is more syntax-heavy than in Haskell
  • lots of popular libraries are not especially functional
  • the DOM is not functional (by design)

However, your last comment -- "I don't want to start putting together my own tool set if it's just going to break everything once it gets too large" -- is a good one. This is basically a problem for every approach, and I can't tell you whether FP in Javascript will be more problematic than "mainstream" techniques when things get too large. But I can tell you that in my experience, FP in Javascript helps me to prevent things from getting too large.

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Great answer; this was exactly what I was looking for. If you have any good further reading on the topic I'd love to hear that, as well.

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