Book Image

Mastering JavaScript Functional Programming

By : Federico Kereki
Book Image

Mastering JavaScript Functional Programming

By: Federico Kereki

Overview of this book

Functional programming is a programming paradigm for developing software using functions. Learning to use functional programming is a good way to write more concise code, with greater concurrency and performance. The JavaScript language is particularly suited to functional programming. This book provides comprehensive coverage of the major topics in functional programming with JavaScript to produce shorter, clearer, and testable programs. You’ll delve into functional programming; including writing and testing pure functions, reducing side-effects, and other features to make your applications functional in nature. Specifically, we’ll explore techniques to simplify coding, apply recursion for loopless coding, learn ways to achieve immutability, implement design patterns, and work with data types. By the end of this book, you’ll have developed the JavaScript skills you need to program functional applications with confidence.
Table of Contents (22 chapters)
Title Page
About the Author
About the Reviewer
Customer Feedback
Connecting Functions - Pipelining and Composition
Answers to Questions

Is JavaScript functional?

About this time, another important question that you should be asking: Is JS a functional language? Usually, when thinking about FP, the mentioned languages do not include JS, but do listless common options, such as Clojure, Erlang, Haskell, or Scala. However, there is no precise definition for FP languages or a precise set of features that such languages should include. The main point is that you can consider a language to be functional if it supports the common programming style associated with FP. 

JavaScript as a tool

What is JS? If you consider popularity indices such as the ones at or, you'll find that JS consistently is in the top ten of popularity. From a more academic point of view, the language is sort of a mixture, with features from several different languages. Several libraries helped the growth of the language, by providing features that weren't so easily available, as classes and inheritance (today's version of JS does support classes, but that was not the case not too long ago) that otherwise had to be simulated by doing some prototype tricks.


The name JavaScript was chosen to take advantage of the popularity of Java — just as a marketing ploy! Its first name was Mocha; then, LiveScript, and only then, JavaScript.

JS has grown to be incredibly powerful. But, as with all power tools, it gives you a way to produce great solutions, and also to do great harm. FP could be considered to be a way to reduce or leave aside some of the worst parts of the language and focus on working in a safer, better way. However, due to the immense amount of existing JS code, you cannot expect large reworkings of the language that would cause most sites to fail. You must learn to live on with the good and the bad, and simply avoid the latter parts.

In addition, JS has a broad variety of available libraries that complete or extend the language in many ways. In this book, we'll be focusing on using JS on its own, but we will make references to existing, available code.

If we ask if JS is actually functional, the answer will be, once again, sorta. JS can be considered to be functional, because of several features such as firstclass functions, anonymous functions, recursion, and closures -- we'll get back to this later. On the other hand, JS has plenty of non–FP aspects, such as side effects (impurity), mutable objects, and practical limits to recursion. So, when programming in a functional way, we'll be taking advantage of all the relevant JS language features, and we'll try to minimize the problems caused by the more conventional parts of the language. In this sense, JS will or won't be functional, depending on your programming style!

If you want to use FP, you should decide upon which language to use. However, opting for fully functional languages may not be so wise. Today, developing code isn't as simple as just using a language: you will surely require frameworks, libraries, and other sundry tools. If we can take advantage of all the provided tools, but at the same time introduce FP ways of working in our code, we'll be getting the best of both worlds — and never mind if JS is or isn't functional!

Going functional with JavaScript

JS has evolved through the years, and the version we'll be using is (informally) called JS8, and (formally) ECMAScript 2017, usually shortened to ES2017 or ES8; this version was finalized in June 2017. The previous versions were:

  • ECMAScript 1, June 1997
  • ECMAScript 2, June 1998, basically the same as the previous version
  • ECMAScript 3, December 1999, with several new functionalities
  • ECMAScript 5 appeared only in December 2009 (and no, there never was an ECMAScript 4, because it was abandoned)
  • ECMAScript 5.1 was out in June 2011
  • ECMAScript 6 (or ES6; later renamed ES2015) in June 2015
  • ECMAScript 7 (also ES7, or ES2016) was finalized in June 2016
  • ECMAScript 8 (ES8 or ES2017) was finalized in June 2017


ECMA originally stood for European Computer Manufacturers Association, but nowadays the name isn't considered an acronym anymore. The organization is responsible for more standards other than JS, including JSON, C#, Dart, and others. See its site at

You can read the standard language specification at Whenever we refer to JS in the text without further specification, ES8 (ES2017) is meant. However, in terms of the language features that are used in the book, if your were just to use ES2015, you'd have no problems with this book. 

No browsers fully implement ES8; most provide an older version, JavaScript 5 (from 2009), with a (always growing) smattering of ES6, ES7, and ES8 features. This will prove to be a problem, but fortunately, a solvable one; we'll get to this shortly, and we'll be using ES8 throughout the book. 


In fact, there are only a little differences between ES2016 and ES2015, such as the Array.prototype.includes method and the exponentiation operator **. There are more differences between ES2017 and ES2016 – such as async and await, some string padding functions, and more – but they won't impact our code.

Key features of JavaScript

JS isn't a functional language, but it has all the features we need to work as if it were. The main features of the language that we will be using are:

  • Functions as firstclass objects
  • Recursion
  • Arrow functions
  • Closures
  • Spread

Let's see some examples of each one, to explain why they will be useful to us.

Functions as First Class Objects

Saying that functions are first class objects (also: first class citizens) means that you can do everything with functions, that you can do with other objects. For example, you can store a function in a variable, you can pass it to a function, you can print it out, and so on. This is really the key to doing FP: we will often be passing functions as parameters (to other functions) or returning a function as the result of a function call. 

If you have been doing async Ajax calls, you have already been using this feature: a callback is a function that will be called after the Ajax call finishes and is passed as a parameter. Using jQuery, you could write something like:

$.get("some/url", someData, function(result, status) {
    // check status, and do something
    // with the result

The $.get() function receives a callback function as a parameter, and calls it after the result is obtained. 


This is better solved, in a more modern way, by using promises or async/await, but for, but for the sake of our example, the older way is enough. We'll be getting back to promises, though, in section Building Better Containers, of chapter 12Building Better Containers – Functional Data Types, when we discuss Monads; in particular, see section Unexpected Monads: Promises .

Since functions can be stored in variables, you could also write:

var doSomething = function(result, status) {
    // check status, and do something
    // with the result
$.get("some/url", someData, doSomething);

We'll be seeing more examples of this in Chapter 6, Producing Functions – Higher–Order Functions, when we consider HigherOrder Functions.


This is a most potent tool for developing algorithms and a great aid for solving large classes of problems. The idea is that a function can at a certain point call itself, and when that call is done, continue working with whatever result it has received. This is usually quite helpful for certain classes of problems or definitions. The most often quoted example is the factorial function (the factorial of n is written n!) as defined for non-negative integer values:

  • If n is 0, then n!=1
  • If n is greater than 0, then n! = n * (n-1)!


The value of n! is the number of ways you can order n different elements in a row. For example, if you want to place five books in line, you can pick any of the five for the first place, and then order the other four in every possible way, so 5! = 5*4!. If you continue to work this example, you'll get that 5! = 5*4*3*2*1=120, so n! is the product of all numbers up to n.

This can be immediately turned into JS code:

function fact(n) {
    if (n === 0) {
        return 1;
    } else {
        return n * fact(n - 1);
console.log(fact(5)); // 120

Recursion will be a great aid for the design of algorithms. By using recursion you could do without any while or for loops -- not that we want to do that, but it's interesting that we can! We'll be devoting the complete chapter 9Designing Functions - Recursion, to designing algorithms and writing functions recursively.


Closures are a way to implement data hiding (with private variables), which leads to modules and other nice features. The key concept is that when you define a function, it can refer to not only its own local variables, but also to everything outside of the context of the function:

function newCounter() {
let count = 0;
    return function() {
return count;
const nc = newCounter();
console.log(nc()); // 1
console.log(nc()); // 2
console.log(nc()); // 3

Even after newCounter exits, the inner function still has access to count, but that variable is not accessible to any other parts of your code.


This isn't a very good example of FP -- a function (nc(), in this case) isn't expected to return different results when called with the same parameters!

We'll find several uses for closures: among others, memoization (see chapter 4, Behaving Properly - Pure Functions, and chapter 6, Producing Functions - Higher-Order Functions) and the module pattern (see chapter 3, Starting Out with Functions - A Core Concept, and chapter 11, Implementing Design Patterns - The Functional Way).

Arrow functions

Arrow functions are just a shorter, more succinct way of creating an (unnamed) function. Arrow functions can be used almost everywhere a classical function can be used, except that they cannot be used as constructors. The syntax is either (parameter, anotherparameter, ...etc) => { statements } or (parameter, anotherparameter, ...etc) => expression . The first one allows you to write as much code as you want; the second is short for { return expression }. We could rewrite our earlier Ajax example as:

$.get("some/url", data, (result, status) =>
    // check status, and do something
    // with the result

A new version of the factorial code could be:

const fact2 = n => {
    if (n === 0) {
        return 1;
    } else {
        return n * fact2(n - 1);
console.log(fact2(5)); // also 120


Arrow functions are usually called anonymous functions, because of their lack of a name. If you need to refer to an arrow function, you'll have to assign it to a variable or object attribute, as we did here; otherwise, you won't be able to use it. We'll see more in section Arrow Functions of Chapter 3, Starting Out with Functions - A Core Concept.

You would probably write the latter as a one-liner -- can you see the equivalence?

const fact3 = n => (n === 0 ? 1 : n * fact3(n - 1));
console.log(fact3(5)); // again 120

With this shorter form, you don't have to write return -- it's implied. A short comment: when the arrow function has a single parameter, you can omit the parentheses around it. I usually prefer leaving them, but I've applied a JS beautifier, prettier, to the code, and it removes them. It's really up to you whether to include them or not! (For more on this tool, check out By the way, my options for formatting were --print-width 75 --tab-width 4 --no-bracket-spacing.


In lambda calculus, a function as x => 2*x would be represented as λx.2*x  -- though there are syntactical differences, the definitions are analogous. Functions with more parameters are a bit more complicated: (x,y)=>x+y would be expressed as λx.λy.x+y. We'll see more about this in section Of Lambdas and functions, in Chapter 3, Starting Out with Functions - A Core Concept, and in section Currying, in Chapter 7, Transforming Functions - Currying and Partial Application.


The spread operator (see lets you expand an expression in places where you would otherwise require multiple arguments, elements, or variables. For example, you can replace arguments in a function call:

const x = [1, 2, 3];
function sum3(a, b, c) {
    return a + b + c;
const y = sum3(...x); // equivalent to sum3(1,2,3)
console.log(y); // 6

You can also create or join arrays:

const f = [1, 2, 3];
const g = [4, ...f, 5]; // [4,1,2,3,5]
const h = [...f, ...g]; // [1,2,3,4,1,2,3,5]

It works with objects too:

const p = { some: 3, data: 5 };
const q = { more: 8, ...p }; // { more:8, some:3, data:5 }

You can also use it to work with functions that expect separate parameters, instead of an array. Common examples of this would be Math.min() and Math.max():

const numbers = [2, 2, 9, 6, 0, 1, 2, 4, 5, 6];
const minA = Math.min(...numbers); // 0

const maxArray = arr => Math.max(...arr);
const maxA = maxArray(numbers); // 9

You can also write the following equation. The .apply() method requires an array of arguments, but .call() expects individual arguments:

someFn.apply(thisArg, someArray) ===, ...someArray);


If you have problems remembering what arguments are required by .apply() and .call(), this mnemonic may help: A is for array, and C is for comma. See and for more information.

Using the spread operator helps write shorter, more concise code, and we will be taking advantage of it.