Learning React, 2nd Edition

Chapter 3. Functional Programming with JavaScript

When you start to explore React, you’ll likely notice that the topic of functional programming comes up a lot. Functional techniques are being used more and more in JavaScript projects, particularly React projects.

It’s likely that you’ve already written functional JavaScript code without thinking about it. If you’ve mapped or reduced an array, then you’re already on your way to becoming a functional JavaScript programmer. Functional programming techniques are core not only to React but to many of the libraries in the React ecosystem as well.

If you’re wondering where this functional trend came from, the answer is the 1930s, with the invention of lambda calculus, or λ-calculus.¹ Functions have been a part of calculus since it emerged in the 17th century. Functions can be sent to functions as arguments or returned from functions as results.  More complex functions, called higher-order functions, can manipulate functions and use them as either arguments or results or both. In the 1930s, Alonzo Church was at Princeton experimenting with these higher-order functions when he invented lambda calculus.

In the late 1950s, John McCarthy took the concepts derived from λ-calculus and applied them to a new programming language called Lisp. Lisp implemented the concept of higher-order functions and functions as first-class members or first-class citizens. A function is considered a first-class member when it can be declared as a variable and ...