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

How do we work with JavaScript?

All this is quite well, but as we mentioned before, it so happens that the JS version available most everywhere isn't ES8, but rather the earlier JS5. An exception to this is Node.js: it is based on Chrome's V8 high-performance JS engine, which already has several ES8 features available. Nonetheless, as of today, ES8 coverage isn't 100% complete, and there are features that you will miss. (Check out for more on Node and V8.)

So, what can you do, if you want to code using the latest version, but the available one is an earlier, poorer one? Or, what happens if most of your users may be using older browsers, which don't support the fancy features you're keen on using? Let's see some solutions for that.


If you want to be sure before using any given new feature, check out the compatibility table at (See Figure 1.1). For Node.js specifically, check out

Figure 1.1. - The latest versions of JS aren't yet widely and fully supported, so you'll have to check before using any of their new features

Using transpilers

In order to get out of this availability and compatibility problem, there are a couple of transpilers that you can use. Transpilers take your original ES8 code, and transform it into equivalent JS5 code. (It's a source-to-source transformation, instead of a source-to-object code as in compilation.) You can code using advanced ES8 features, but the user's browsers will receive JS5 code. A transpiler will also let you keep up with upcoming versions of the language, despite the time needed by browsers to adopt new standards across desktop and mobile devices.


If you wonder where did the word transpiler come from, it is a portmanteau of translate and compiler. There are many such combinations in technological speak: email (electronic+mail), emoticon (emotion+icon), malware (malicious+software), or alphanumeric (alphabetic+numeric), and several more.

The most common transpilers for JS are Babel (at and Traceur (at With tools such as npm or Webpack, it's fairly easy to configure things so your code will get automatically transpiled and provided to end users. You can also try out transpilation online; see Figure 1.2 for an example using Babel's online environment:

Figure 1.2 - The Babel transpiler converts ES8 code into compatible JS5 code

If you prefer Traceur, use its tool at instead, but you'll have to open a developer console to see the results of your running code. (See Figure 1.3 for this.) Select the EXPERIMENTAL option, to fully enable ES8 support:

Figure 1.3 - The Traceur transpiler is an equally valid alternative for ES8-to-JS5 translation


Using transpilers is also a great way to learn the new JS features. Just type in some code at the left, and see the equivalent result at the right. Alternatively, use command line interface (CLI) tools to transpile a source file, and then inspect the produced output.

There's a final possibility you may want to consider: instead of JS, opt for Microsoft's TypeScript (at, a superset of JS, compiled to JS5. The main advantage of TypeScript is adding (optional) static type checks to JS, which helps detect some programming errors at compile time. Beware: as with Babel or Traceur, not all of ES8 will be available.


You can also get type checks, without using TypeScript, by using Facebook's Flow (see

If you opt to go with TypeScript, you can also test it online at their playground; see You can set options to be more or less strict with data types checks, and you can also run your code on the spot. See figure 1.4:

Figure 1.4 - TypeScript adds type checking features, for safer JS programming

Working online

There are some more online tools that you can use to test out your JS code. Check out JSFiddle (at, CodePen (at, or JSBin (at, among others. You may have to specify whether to use Babel or Traceur; otherwise, newer JS features will be rejected. See an example of JSFiddle in Figure 1.5:

Figure 1.5 - JSFiddle lets you try out ES8 code (plus HTML and CSS) without requiring any further tools


We will also touch on testing, which is, after all, one of FP's main advantages. For that, we will be using Jasmine (, though we could also opt for Mocha (

You can run Jasmine test suites with a runner such as Karma (, but I opted for standalone tests; see for details.