Functional Programming in JavaScript? Yes Please.

3 min read

One of the hot topics right now in the web development world is functional programming in the language of the web, JavaScript.

Functional programming encompasses a whole host of mathematical properties and phenomena that is beyond this post, but what I am going to address here is how to write a a few functions with nominal functional programming.

This is going to be a series. I am currently researching these topics as I go along and what I find excites me. I will be digging into these topics, each array method bit by bit. You can find some more of this on Steve Smith’s blog Funky Javascript.

Important Concept: Higher Order Functions

One of the best and worst parts about JavaScript is that you can pass functions into other functions. This can lead to beautifully expressive code and sometimes bugs.

Why can we do this? Because, like in most functional programming languages 1, functions are values just like any other value in JavaScript.

Take this code:

// Named function or
// function declaration
function double(x) {
  return x * 2;

// Anonymous function or
// Function expression
let double = function(x) {
  return x * 2;

let cat = double;
// Function call

// returns 120, obviously

Here we have named function called double. It takes an argument, x and when you call it, it returns whatever the value of x is that you specify in the function call and returns it.

What’s different about JavaScript is that you can pass it into a variable and call the function from that variable. This is because, well, functions are values.

Higher order functions are good for composition to take smaller functions and make them into bigger functions. More on this in a minute.

Enter .filter()

The .filter() function is an array function that takes a callback that it uses to create a new filtered version of an array.

Take this array of objects:

let animals = [
  { name: ‘Catticus Finch’, species: ‘cat’ },
  { name: ‘Peaches’ species: ‘fish’ },
  { name: ‘Bobby’, species: ‘dog’ },
  { name: ‘Lucifer’, species: ‘cat’ },
  { name: ‘Beatrix’, species: ‘rabbit’ },
  { name: ‘Cerulean’ species: ‘fish’ }

Say I wanted to filter out all this cats in this array. We could use the trusty for loop:

let cats = []

for (let i = 0; i < animals.length; i++) {
  if (animals[i].species === ‘cat’) {

We are essentially just looping through the array and for every cat the for loop finds, it pushes it into the empty cat array.

Now, we can filter.

Filter accepts a callback and loops through each item in the array and passes it back to the callback function. .filter() expects a boolean and then returns the filtered array.

In ES5:

let cats = animals.filter(function(animal) {
  return animal.species === ‘cat’;

In ES6

let cats = animals.filter(animal => animal.species === ‘cat’);

Here, if the value of the species property in the animals array is a cat it will return the names of those cats in a filtered array.

We could also write an anonymous function and add a filter function inside of it, much like this:

let isCat = function(animal){
  return animal.species === ‘cats’;

let cats = animals.filter(isCat);

How Cool is This?

Writing small functions allows composition which we can reuse. In the for loop, we are pushing cats into the array but filter handles this natively.

Filter and the callback are composed as they are meshed into each other. Not only is it syntactically pleasing, but it is less lines of code which is always a good thing.

Next Up

Next I want to tackle the .map() function and get back to my Chrome DevTools series.

  1. Though JavaScript isn’t a purely functional programming language. 

CC BY-SA 4.0 Functional Programming in JavaScript? Yes Please. by Tiffany White is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.