JS Functional Programming Libraries

June 13, 2015

##In computer science, Functional Programming (“FP”) is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data.

Shortly, FP is a coding style based on the following two best practices, which together form the concept of functions purity:

  1. Immutability and
  2. Statelessness

In JavaScript, we have plenty of libraries to choose from with a functional flavor. While each has its set of goals, in the following paragraphs we’ll see which are the ones we should all know about and why.


Underscore.js is a utility-belt library for JavaScript that provides support for the usual functional suspects (each, map, reduce, filter…) without extending any core JavaScript objects.

Lodash is a JavaScript utility library delivering consistency, modularity, performance, & extras.

Why should we care

As of today, lodash is the most dependended-upon package on npm while underscore is the third most dependended-upon.

Lodash in numbers (impressive):

  • 500,000+ daily downloads
  • 13,000,000+ monthly downloads
  • 11,300+ depended-upon packages

Underscore in numbers (still pretty impressive):

  • 300,000+ daily downloads
  • 6,900,000+ monthly downloads
  • 9,400+ depended-upon packages

Things worth knowing about underscore


Underscore comes with a chain method. It allows us to use _.chain() to wrap a collection and then call other underscore functions on it before finally calling _.value() to unwrap the final result.

Taking a movies collection:

var movies = [
  { title: "Mad Max: Fury Road", ratings: [8.9, 9.9, 7.9], year: 2015 },
  { title: "Tomorrowland", ratings: [6.0, 7.0, 8.0], year: 2015 },
  { title: "Jurassic World", ratings: [7.2, 8.2, 9.2], year: 2015 },
  { title: "Inception", ratings: [7.5, 7.5, 8.5], year: 2010 }

and some basic helpers

var sum = function(numbers) {
    return _.reduce(numbers, function(sum, n){
        return sum + n }, 0);

var average = function (numbers) {
    return sum(numbers) / numbers.length;

var isRecommended = function (movie) {
    return average(movie.ratings) >= 8;

We can now create a complex operation by simply chaining together underscore functions. For example, to discover the lowest rating ever given to a recommended movie:

// → 7.2


If _.chain seems well known and commonly used, fewer may be aware of an underscore hidden gem: compose. Compose is as close as we can get to function composition within underscore, by allowing us to returns the composition of a list of functions, where each function consumes the return value of the function that follows.

In math terms, composing the functions f(), g(), and h() produces f(g(h())).

Given some generic functions

var greet = function(name){
    return "hi: " + name;

var exclaim  = function(statement){
    return statement.toUpperCase() + "!";

We can now _.compose them to create a more powerful function out of the two:

var welcome = _.compose(greet, exclaim);
// → hi: THERE!

Note: compose invokes the provided functions from right to left.

Things worth knowing about lodash

Composition left

Lodash is born as a fork of underscore. Therefore, everything we just saw can also be achieved with lodash. For example, we may like compose but it may feel weird for its right to left math derived invocation. Lodash still permits that mathematical approach but also features a left-to-right version of the method simply called _.flow.

Lodash, representing underscore gen2 offer better performances thanks to advanced code optimizations. It can act as a drop-in replacement for underscore, and it won’t be hard to replace the first with the latter.

Among the extra perks that come with lodash one is worth exploring:


In JavaScript a function takes some arguments and returns a value. We can call a function with:

  • fewer parameters then the function’s arity (arguments.length<function.length), most probably with odd results, or
  • with too many, exceding the function’s arity (arguments.length>function.length) with the result that the arguments in excess normally get ignored.

A curried function is a function that may be invoked with fewer arguments than its original arity, returning a function ready to be invoked with the remaining ones.

Given a sum3 function, that when invoked with three arguments will return their sum, we can create its curried version as follows:

var sum3 = function(a, b, c) {
  return a + b + c;

var curriedSum = _.curry(sum3);

// → 6

curriedSum(1, 2)(3);
// → 6

curriedSum(1, 2, 3);
// → 6

// using empty placeholders
curriedSum(1)(_, 3)(2);
// → 6

If you love the taste of curry (and you should, it’s damn tasty) then you’ll probably need to know about Ramda.


Ramda is a practical functional library for Javascript programmers.

Ramda is not a drop-in replacement for underscore or lodash, as it has a more focused goal. It’s a library designed specifically for a functional programming style, one that makes it easy to create functional pipelines, one that never mutates user data.

Things worth knowing about Ramda

Function first, data last API

The parameters to Ramda functions are arranged to make it convenient for currying as the data to be operated on is supplied last.

lodash map vs. rampda map functions

// lodash
_.map(collection, iteratee)

// Ramda
R.map(iteratee, collection)


Ramda functions are automatically curried. Currying allows you to easily build up new functions from old ones simply by not supplying the final parameters. This coupled with the function first, data last API make ramda not just functional purer, but also purely awesome.

Let’s then rebuild part of the underscore/lodash code we saw earlier with ramda instead.

// Underscore/lodash style:
var recommendedMovies = function(movies) {
  return _.filter(movies, isRecommended);

// Ramda style:
var recommendedMovies = R.filter(isRecommended);

Because every ramda function automatically curry, omitting to pass the collection argument to R.filter return us the curryed function so that we are ready to go.

Other previously encountered code re-factored with Ramda:

// Underscore:
// → 7.2

// lodash:
// → 7.2

// Ramda:
// → 7.2
// Spot something? Yes pipe return a function.

Is worth mentioning that there is a version of lodash that comes packed with functionFirst dataLast APIs and auto-currying, called lodash-fp. Let’s see how that could be beneficial, for example within a composition (flow) taken from the example we just saw:

// lodash flow
// passing anonimous functions
  function(list){return _.filter(list, isRecommended);},
  function(list){return _.pluck(list, 'ratings');},

// lodash flow
// passing named functions
var customfilter = function(list){return _.filter(list, isRecommended);};
var custompluck = function(list){return _.pluck(list, 'ratings');};


// lodash flow
// passing curried functions
var curriedfilter = _.curry(_.filter)(_,isRecommended, null)
var curriedpluck = _.curry(_.pluck)(_, 'ratings')


// lodash-fp flow
// Function-first + Auto-curry awesomeness in action

We now know why we should care about some functional libraries, how to do chaining, composition and currying with them. For more info check their respective documentation:

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