Functional JavaScript

Chapter 2. First-Class Functions and Applicative Programming

The basics of functional programming, which treats functions as first-class elements of a language, are covered in this chapter. I’ll provide a basis in the three common functions: map, reduce, and filter. Since programmers will likely be familiar with these functions, using them as a starting point should provide a nice foundation for the rest of the book.

Functions as First-Class Things

Some programmers familiar with JavaScript, myself included, consider it to be a functional language. Of course, to say such a thing implies that others disagree with that assessment. The reason for this disagreement stems from the fact that functional programming often has a relative definition, differing in minor and major ways from one practitioner or theorist to another.^[25]

This is a sad state of affairs, indeed. Thankfully, however, almost every single relative definition of functional programming seems to agree on one point: a functional programming language is one facilitating the use and creation of first-class functions.

Typically, you will see this point accompanied by other definitional qualifications including but not limited to static typing, pattern matching, immutability, purity, and so on. However, while these other points describe certain implementations of functional programming languages, they fail in broad applicability. If I boil down the definition to its essence, consisting of the terms “facilitating” and “first-class ...