Functional programming in a nonfunctional world

Can functional and nonfunctional programming be mixed together? Although this is the subject of , Functional & Object-oriented Programming in JavaScript, it is important to get a few things straight before we go any further.

This book is not intended to teach you how to implement an entire application that strictly adheres to the rigors of pure functional programming. Such applications are rarely appropriate outside Academia. Rather, this book will teach you how to use functional programming design strategies within your applications to complement the necessary imperative code.

For example, if you need the first four words that only contain letters out of some text, they could naively be written like this:

var words = [], count = 0; text = myString.split(' '); for (i=0; count<4, i<text.length; i++) {   if (!text[i].match(/[0-9]/)) {     words = words.concat(text[i]);     count++;   } } console.log(words);

In contrast, a functional programmer might write them as follows:

var words = []; var words = myString.split(' ').filter(function(x){   return (! x.match(/[1-9]+/)); }).slice(0,4); console.log(words);

Or, with a library of functional programming utilities, they can be simplified even further:

var words = toSequence(myString).match(/[a-zA-Z]+/).first(4);

The key to identifying functions that can be written in a more functional way is to look for loops and temporary variables, such as words and count instances in the preceding example. We can usually do away with both temporary variables and loops by replacing them with higher-order functions, which we will explore later in this chapter.

Is JavaScript a functional programming language?

There is one last question we must ask ourselves. Is JavaScript a functional language or a non-functional language?

JavaScript is arguably the world's most popular and least understood functional programming language. JavaScript is a functional programming language in C-like clothing. Its syntax is undeniably C-like, meaning it uses C's block syntax and in-fix ordering. And it's one of the worst named languages in existence. It doesn't take a lot of imagination to see how so many people can confuse JavaScript as being related to Java; somehow, its name implies that it should be! But in reality it has very little in common with Java. And, to really cement the idea that JavaScript is an object-oriented language, libraries and frameworks such as Dojo and ease.js have been hard at work attempting to abstract it and make it suitable for object-oriented programming. JavaScript came of age in the 1990s when OOP was all the buzz, and we've been told that JavaScript is object-oriented because we want it to be so badly. But it is not.

Its true identity is much more aligned with its ancestors: Scheme and Lisp, two classic functional languages. JavaScript is a functional language, all the way. Its functions are first-class and can be nested, it has closures and compositions, and it allows for currying and monads. All of these are key to functional programming. Here are a few more reasons why JavaScript is a functional language:

JavaScript's lexical grammar includes the ability to pass functions as arguments, has an inferred type system, and allows for anonymous functions, higher-order functions, closures and more. These facts are paramount to achieving the structure and behavior of functional programming.
It is not a pure object-oriented language, with most object-oriented design patterns achieved by copying the Prototype object, a weak model for object-oriented programming. European Computer Manufacturers Association Script (ECMAScript), JavaScript's formal and standardized specifications for implementation, states the following in specification 4.2.1:
"ECMAScript does not contain proper classes such as those in C++, Smalltalk, or Java, but rather, supports constructors which create objects. In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behavior. In ECMAScript, the state and methods are carried by objects, and structure, behavior and state are all inherited."
It is an interpreted language. Sometimes called "engines", JavaScript interpreters often closely resemble Scheme interpreters. Both are dynamic, both have flexible datatypes that easily combine and transform, both evaluate the code into blocks of expressions, and both treat functions similarly.

That being said, it is true that JavaScript is not a pure functional language. What's lacking is lazy evaluation and built-in immutable data. This is because most interpreters are call-by-name and not call-by-need. JavaScript also isn't very good with recursion due to the way it handles tail calls. However, all of these issues can be mitigated with a little bit of attention. Non-strict evaluation, required for infinite sequences and lazy evaluation, can be achieved with a library called Lazy.js. Immutable data can be achieved simply by programming technique, but this requires more programmer discipline rather than relying on the language to take care of it. And recursive tail call elimination can be achieved with a method called Trampolining. These issues will be addressed in , Advanced Topics & Pitfalls in JavaScript.

Many debates have been waged over whether or not JavaScript is a functional language, an object-oriented language, both, or neither. And this won't be the last debate.

In the end, functional programming is way of writing cleaner code through clever ways of mutating, combining, and using functions. And JavaScript provides an excellent medium for this approach. If you really want to use JavaScript to its full potential, you must learn how to use it as a functional language.