Functional Fashion in JavaScript

Functional programming in JavaScript

Blog Functional Fashion in JavaScript

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It’s funny how fashion works, most people 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.

In technology same things happen, everyone wants to be fashionable. There are 2 users of technology fashion: the final consumer, who usually has little technical knowledge and who doesn’t care about the components of their product as long as it’s 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"

What people is talking about.

We already know that JavaScript is a famous language and its functional version is a fashionable subject. But it’s 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’re based on the fact that JavaScript is a multi-paradigm language and therefore it’s self-sufficient in a functional way.

Our opinion

In Fluid Attacks we believe that use of functional JavaScript doesn’t 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 cannot 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 cannot use 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 don’t 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’s 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:

Cylinder class example.

class cylinder {
  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.

Cylinder class in a functional approach.

function cylinder(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’s 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’s something difficult to understand the first time but I’ll explain it with an example.

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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’s 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 don’t expect you to become an eminence in functional JavaScript, but we want you to understand what everyone is talking about. We’ll 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. As the queen in this dance is JavaScript, we’ll introduce an extension to a well-known linter of this language, which will allow us to enable rules that ensure that we’re 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 (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, so I invite you to understand what each one is for here. But as an appetizer we’ll talk about some rules and we’ll 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 shouldn’t 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 cylinder 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’s 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.

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’s better to be fashionable because we don’t 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!

P.S. What about the security of your code? Have you read our main blog post on secure code review?

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