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A man with a functional JavaScript t-shirt

Functional Fashion in JavaScript

Functional programming in JavaScript
What are the new trends in the JavaScript world. First steps in functional JavaScript. Recommendations for the migration from any paradigm to functional. How to use eslint and its plugin for checking functional code. Functional JavaScript: fleeting fashion or imminent future?

It’s funny how fashion works, most people (if not all) want to be fashionable but just a bit understand where the fashion comes from. Fashion pieces are not understood but still people like to use them. For example, if you have ever seen a fashion show or any event like that, you may have noticed that they are very extravagant: feathers, flowers, colors, plastic and bright things. Nobody wears those suits in their daily lives. So, why do people meet in Milan, New York, Paris and London? Basically, it is for setting the new trends. First mistake trying to understand the fashion shows is to see the clothes as a whole, what really should be done is to look at the details separately: what color are they using?, what textures?, what materials?, etc. In technology same things happens, everyone wants to be fashionable. There are 2 users of technology fashion: the final consumer, who usually has little technical knowledge and who does not care about the components of their product as long as it is the latest in the market. Second one is behind creating the product, a person with technical knowledge who incorporate new technological trends in his products. One of the latest fashion trends is functional JavaScript. JavaScript is a very popular language because of the ubiquity of web browsers. If you have a modern browser then you have a JavaScript console at your disposal.

Divided opinions

Two people taking in a theater
Figure 1. What people is talking about.

We already know that JavaScript is a famous language and its functional version is a fashionable subject, big and small ones are talking about that. But it is a controversial issue, which has well-defined extremes: the reluctants and the totalitarians. The first ones say that JavaScript is a language that mandatorily needs the states to reach their full potential in terms of web pages and applications. Reluctants add that there are functions like foreach that are necessary for an optimal code. On the other side, totalitarians think in a functional world, all JavaScript without a single trace of objects. They are based on the fact that JavaScript is a multi-paradigm language and therefore it is self-sufficient in a functional way.

Our opinion

In Fluid Attacks we believe that use of functional JavaScript does not have to be white or black, but a scale of grays that we can adapt to make it more light or dark. JavaScript has its well-defined bases and we can not go completely against its nature, many libraries have evolved over time to be more efficient. Nor can we stay behind, so we must improve. We can not use (for now) a fully functional JavaScript but we can code with a functional style. This is why our way is to implement functional approaches step by step. There are issues that are difficult to implement but this may be different in the future. Let’s change something here and then change something there but do not try to change everything in one shot or less start from scratch.

How to be fashionable?

To know what we must change to be functional, it is enough to read what is functional programming and what are its cornerstones (we recommend you to read Why we go functional?). This paradigm is against classes because this structures stimulate the use of states and its mutations. For example the following class:

Cilinder class example.
class cilinder {
  constructor(radius, height) {
    this.radius = radius;
    this.height = height;

This is the common way to make a class, it makes sense to do so if what you want is a replicable object. Now we rewrite this code in a functional way.

Cilinder class in a functional approach.
function cilinder(radius, height) {
  return {
    "radius": radius,
    "height": height

As you can see, JavaScript classes can be represented as functions. If you think your classes are too large to be represented as functions, then you should first divide those classes into granular classes that do only one thing.

Another stone in the shoe when trying to migrate from object-oriented to functional are the iterative cycles. Let’s look at the following code:

Increment in list example.
const result = [];
const integers = [1, 2, 3];

for (let i = 0; i < elements.length; i++) {
  result.push(elements[i] + 1);

This code adds a one to all the elements of a list. Let’s represent this cycle in a functional way:

Increment in list example, functional approach.
const result = [];
const integers = [1, 2, 3];
const result = => element + 1);

Code is not only more compact but it is also easier to understand. Like Python, JavaScript is multi-paradigm, which means that there are a number of built-in functions that allow your code to be functional without the need to import any additional library. Some of these functions are map, filter and reduce.

A final detail we want to show you, a small introduction to high-order functions in JavaScript, something very characteristic in the functional paradigm, there are functions that can receive functions as a parameter and can even return other functions. I know, it is something difficult to understand the first time but I will explain it with an example.

Multiply function is JavaScript.
var multiply = function(a, b) {
  return a * b;

multiply(2,3); // 6

The above is a classic function to multiply 2 integers. Now let express this in a high-order function.

Multiplication in high-order function.
function multiplyByInteger(a) {
  return function(b) {
    a * b

var multiplyByTwo = multiplyByInteger(2);
multiplyByTwo(3); // 6

As you can see our multiplyByInteger function returns another function. The new function only requires an argument to perform multiplication, this is known as partial evaluation.

With this article we do not expect you to become an eminence in functional JavaScript, but we want you to understand what everyone is talking about. We will try to convince you to follow the recommendations we made in this article.

Poorly dressed guests

Keeping a functional mind while migrating from any paradigm can be a bit difficult. Old habits are hard to remove. Our brain constantly invites us to safety, to use something we already know and know it works. When we think we have written functional code, it may not be like that. Even the code may look functional but it really is a wolf in sheep’s clothing, its essence is still object-oriented.

Fashion police

Thanks to the programming gods, there are tools that allow us to check if our code is really functional, that is, it allows us to discover the wolves amongst sheeps. As the queen in this dance is JavaScript, we will introduce an extension to a well-known linter of this language, which will allow us to enable rules that ensure that we were programming in functional style. We will use eslint-plugin-fp.

Assuming that you already have eslint, we proceed to install the plugin:

eslint-plugin-fp installation.
npm install --save eslint-plugin-fp

Now we must enable the plugin and add the rules we want to check, therefore, we must modify the eslint configuration file (it is usually a JSON type file) adding the new library to the plugin list and the new rules to the list of rules.

Part of the eslint configuration part.
"plugins": [
"rules": {
  "fp/no-arguments": "error",
  "fp/no-class": "error",
  "fp/no-delete": "error",
  "fp/no-events": "error",
  "fp/no-get-set": "error",
  "fp/no-let": "error",
  "fp/no-loops": "error",
  "fp/no-mutating-assign": "error",
  "fp/no-mutating-methods": "error",
  "fp/no-mutation": "error",
  "fp/no-nil": "error",
  "fp/no-proxy": "error",
  "fp/no-rest-parameters": "error",
  "fp/no-this": "error",
  "fp/no-throw": "error",
  "fp/no-unused-expression": "error",
  "fp/no-valueof-field": "error",

As you can see the rules are too many to talk about all of them, so I invite you to understand what each one is for here. But as an appetizer we will talk about some rules and we will set an example so you can see how this plugin works.

The no-class rule avoids the use of classes, if you have been reading carefully, you know why the classes should not go in a functional approach but if you are a clueless I remind you that the classes are full of states.

Let’s put the cilinder class example in a file called example.js and use eslint with the no-class rule to observe what happens.

Linting example.js.
$ eslint example.js


  1:1  error  Unallowed use of `class`. Use functions instead  fp/no-class

✖ 1 problem (1 error, 0 warnings)

An error is displayed saying which is the rule that is being violated, in which line and in which file. Remember that, it is best to run this linter in your CI to ensure that the code will not be deployed to production without first passing all the tests.


We remind you that if you already have a lot of code written in JavaScript with some paradigm that is not functional and you want to start migrating, the best way to start is by converting small fragments of code into functional ones. At the beginning it may take a while but you will earn a lot of expertise. Speculating about the future is very difficult, in general the functional paradigm has taken place in all programming languages ​​in recent years, but some experts consider that everything is a fashion subject and at some point, functional boom will end. Nobody really knows what to say about this. But in technology it is better to be fashionable because we do not know at what time old things will stop working, falling into oblivion. JavaScript still has a lot to give, there are already JavaScript-based frameworks that are functional. What a good time to be fashionable!

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Oswaldo José Parada Cuadros

Mechanical Engineer

Family, friends and little details. There is the answer.


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