JavaScript Functional Programming: Introduction

As with fashion, in programming there are trends that come and go. Some have die hard fans, sometimes this trends are just hype and never get used… And some times they reappear.

You could say that functional programming is one of those trends… Please don’t get mad at me for saying that, I’m just trying to make a point.

The point being that functional programming is NOT THE way to develop, nor its a new technology. Functional programming its just a way to develop that has great benefits and some caveats.

Recently, with the advent of React Hooks, it seems that Functional Programming is again in the spotlight. So let’s take a look at what does it mean that JavaScript is functional and what are the benefits.

This is the first part of a small mini series where I’ll be discussing functional programming in JavaScript. On part 2 I’ll dig deeper on the core concepts of functional programming. An in part 3 I’ll introduce some programming patterns that can be used with functional programming like Composition and Currying.

If you want a more in depth introduction made by someone much more smarter than me, head to the article Composing Software by Eric Elliot (look towards the end).


The beginnings of functional programming

Let’s start by saying that functions have been a part of calculus since the 17th century. And if you went trough high school, you might have seen something like

f(x) = y

That’s a way to represent a function (f()). Something that receives an input (x), and generates an output (y). And that what functions are… Something that receives zero or more parameters and returns a result!

Well, in 1930 Alonzo Church, at Princeton University, invented lambda calculus or λ-calculus while experimenting with higher order functions.

Higher-order functions are functions that can manipulate other functions and use them either as arguments, results or both. This is important, well see why latter.

Then, in the late 1950’s, John McCarty took the concepts derived from λ-calculus and applied them to a new programming language called Lisp.

In Lisp, functions are a first-class citizens. Which means that function can be declared as variables and send to functions as a parameters. They can even be the return of a function (functions than when executed returns another function).

Is JavaScript a “functional language”?

JavaScript its a (non exclusively) functional programming language because in JavaScript functions are first-class citizens.

But what does it mean that functions are first class citizens?

1. Variables can be functions

// functions-as-vars.js
const varFn = function(message) {
  return console.log("Hello " + message)
varFn("lambda function")

Will return

Hello lambda function

Here the variable varFn its actually a function.

2. Functions can be added to objects like variables

// functions-in-objects.js
const obj = {
  prefix: "Hello",
  showMessage(message) {
    console.log(this.prefix + " " + message)

Will print

Hello world

They work very similar to classes, but if you look closely, you can see that we added the function to the object while creating the object. We didn’t need to create a Class and then instantiate it with new.

3. Functions can be passed as parameters

// functions-as-parameters.js
function executeLogger(loggerFunction) {
  loggerFunction("Inside function as param")
function logger(message) {
  console.log("Hello - " + message)

Will output

Hello - Inside function as param

Here executeLogger is a higher order function. It receives functions as parameters.

Told you that it was important, didn’t I? 😉.

4. Functions can return functions

// functions-ret-functions.js
const loggerFn = function(message) {
const toUpperFn = function(logger) {
  return function(message) {
    logger("The message is: " + message.toUpperCase())
// `loggerToUpperFn` receives a function than can be executed
const loggerToUpperFn = toUpperFn(loggerFn)
loggerToUpperFn("hola mundo")

Will output

The message is: HOLA MUNDO

If you are familiar with ES6. You can infer that this last snippet can be rewritten like this:

// functions-ret-functions-es6.js
const toUpperFn = logger => message =>
  logger("The message is: " + message.toUpperCase())

toUpperFn(message => console.log(message))("hola mundo")

Imperative vs Declarative programming

Good, we now know some principles of functional programming and how they get implemented in JavaScript. But we still don’t know how to use them to write better programs.

For that we have to make the distinction between Imperative vs Declarative programming.

Imperative programming

Imperative Programming focuses on explaining which steps should happen to get a result. In other words, it focuses on how things should happen. It can be said that this is the traditional way of creating software.

// imperative-programming.js
const message = ["this is", "imperative programming", "!"]
let transformed = ""
for (let i = 0; i < message.length; i++) {
  let item = message[i].toUpperCase()
  item = item.replace(/ /g, "-")
  transformed = transformed + "-" + item

Will output


In the previous code, you can see how the code gets to the result. Each step is clear an concise, but is not clear what each step achieves. Only after going trough the whole code is clear what we’ve done.

Declarative Programming

Declarative Programming in the other hand, is a style of programming where applications are structured in a way that prioritizes what should happen (given that you use clear function names).

Functional programming is part of declarative programming.

If we where to rewrite that previous code declaratively, we should do something like:

// declarative-programming.js
const makeUppercase = msg => msg.toUpperCase()
const hypenize = msg => msg.replace(/ /g, '-')
const appendTo = msg => accum => accum + '-' + msg
const pipe = (...fns) => x => fns.reduce( (res, fn) => fn(res), x)

const message = ['this is', 'declarative programming', '!']

const transformed = message.reduce( (item, accum) =>


Right now is not important to understand the code. The important thing is to show how the code is written.

The previous code will output:


Now, this may seem like an overkill since we’re doing very little logic but when we’re working with a 10.000 line project is a blessing since the code is basically telling us what the code does:

So you can see what the code does, but no how it does it. That’s almost the definition of declarative programming.

Why use functional programming?

You might be thinking from the previous example that functional programming just makes thing harder since you have to think what the code should do before actually writing.

And you might not be completely wrong.

Functional programming forces you to think MORE before writing your software. But that could be taken as a good thing.

Additionally there are good reasons to use functional programming:

That last one is a little controversial since it depends on the developer, and is NOT ALWAYS TRUE. Actually, Functional programming can be more inefficient and memory intensive if you are not careful.

Some words about React

If after reading all of this you are wondering if this is actually useful. So let’s look at React and how it uses JXS to represent content in the DOM:

// react-declarative-jsx.js
import React from "react"
import ReactDOM from "react-dom"

const HelloWorld = () => {
  const [msg, setMsg] = useState("Hello World");
  return (
    <div className="container">
ReactDOM.render(<HelloWorld />, document.getElementById("root"))

Right of the bat, you can see that the code is very declarative. It’s very obvious what it should be outputted with out analyzing step by step how it does it.

And to top it, since React introduced hooks in version 16, they did added a declarative way to manage state in the framework (I know is not a framework but it almost is).

So if you are a React developer, you should have a good grasp of functional programming. It not only allow you to understand the code better, but it will make you code better at the end.

On part 2 of this mini series I’ll be going over some concepts like Immutability, Purity, and Recursion to finally understand what functional programming works.

Finally, you can browse the code in this blog on this GitHub repo