1. Functional programing (FP)

Functional programming is a programming paradigm, that is another, different than OOP, way of thinking about how to build the application. The main unit of functional programming is a function. We already use functions on daily basis so what is the difference in this case? To use FP we have to think differently, the aim is to abstract control flows and operations on data with functions, so by doing that we could avoid side effects and reduce mutation of state.

1.1 Imperative programing paradigm

Imperative programming is one of the oldest programming paradigms. It has a close connection to machine architecture. By changing the state through assignment statements we are achieving results. The state is changing by performing step-by-step tasks. The main concept is how to achieve our goal. If you are following the imperative programming paradigm you have several statements and the result is stored after execution of all of them.

Pros

• Very simple to implement

Cons

• Less efficient and less productive
• Parallel programming is not possible

Listing 1.1

const doubleMap = (numbers) => {
  const doubled = [];

  for (let i = 0; i < numbers.length; i++) {
    doubled.push(numbers[i] * 2);
  }

  return doubled;
};

console.log(doubleMap([2, 3, 4])); // [4, 6, 8]

In the Listing 1.1 our goal is to double each array element and return a new array with doubled values. As was mentioned before, in case of imperative programming, our focus is on HOW to achieve the result. So, we have several main steps:

• creation of new empty array doubled
• going through array with for loop
• double each element
• push the doubled element into doubled array
• return doubled array

By doing these steps one-by-one as a result we will have [4, 6, 8].

1.2 Declarative programing paradigm

The main idea of Declarative programming paradigm is to define what needs to be accomplished by the program, but not how it needs to be implemented. So, in other words, instead of instructing how to achieve the desired results we focus only on the result itself. It is different from imperative programming which focuses on a set of commands which need to be executed in order to achieve the required solution. Declarative programming describes a particular class of problems that have to be solved and a language implementation takes care of finding the solution. With this approach, the resulting program is simpler to read. The same example, but in a declarative way.

Listing 1.2

const doubleMap = (numbers) => numbers.map((n) => n * 2);

console.log(doubleMap([2, 3, 4])); // [4, 6, 8]

Let's analyze what is the difference - in this case, we know that map function creates a new array, so we do not need to think about it. We also know that map goes through the array, so we can skip the implementation of these parts.

So, there are several main steps:

• all routine work is done by map
• we only pass the callback in which we provide the condition which is important to us in this case.

1.3 Is FP imperative or declarative?

In order to answer this question, let's compare these two approaches:

Characteristic	Imperative Approach	Functional Approach
Description	The program directly changes computed state	The program avoids mutating state and computation data
Key points	Direct assignments, global variables, common data structures	Compositiionality, resursion, no side effects
Programmer focus	How to perform tasks and how to track state changing	What is desired and what transformations are required
State changes	Important	Non-existent
Order of execution	Important	Low importance
Primary flow control	Loops, conditionals and function calls	Function calls, resursion
Primary manipulation unit	Instances of structures or classes	Functions as first-class objects and data collections

As we can see FP implements most of the declarative rules, such as programmers focus on what to do, composition, recursion, immutability, functions as first-class objects, etc.