Introduction

Functional programming is a programming paradigm that treats computations as evaluations of mathematical functions, avoiding state changes and mutable data.

This is a declarative programming style that focuses on “what to solve” rather than an imperative style that focuses on “how to solve”. It uses expressions Instead of statements.

While the statement is being executed to assign the variable, the expression is evaluated to produce the value.

Key Concepts

Pure Functions:

Pure functions are a fundamental concept of functional programming.It is a type of function that follows certain principles, so it is predictable, reliable, and easy to understand.

Key attributes of pure functions are:

Deterministic:

A pure function produces the same output for the same set of input parameters each time it is called. It does not depend on external factors such as global variables or system states that can cause randomness and unpredictability.

No side effects

This function does not cause any side effects. Side effects can include I/O (such as writing to the console or log files), modifying mutable objects, reassigning variables, and so on. This works using only input parameters.

_To understand better, consider the following example._

function add(a,b) {
  return a + b}
Console.log(add(9,6))

This example includes a simple add() function that returns 15 as output.

This is a very predictable output that doesn’t depend on any external code.

This makes the add() function a pure function.

Immutability

Immutability means the state of not changing, or being unable to be changed. In functional programming, data is immutable.

The data structure cannot be modified once created.

If you want to modify an immutable object, create a new object with the new value.

To understand better, consider the following example.

Const arr = [5, 4, 3, 2];
Const newArr = […arr, 1];
// ‘newArr’ is [5, 4, 3, 2, 1], ‘arr’ is still [5, 4, 3, 2]

As you can see in the example above, the original array is unchanged.

Instead, a new array is created based on the old array.

The original array is still available and can be used elsewhere in the program if needed.

First-class functions

Functional programming treats functions as first-class citizens.

That is, functions can be passed as arguments to other functions, returned as values from other functions, assigned to variables, and stored in data structures.

Functions as Values

Functions can be assigned to variables like any other data type.

This means that functions can be summarized within functions and documented by name, allowing for cleaner, more modular code.

**Example**:

const greet = function (name) {

   return ‘Hello, ${name}:’ ;
   };

Functions as arguments:

Functions can be passed as arguments to other functions.

This concept enables the creation of higher-order functions that work together with other functions, providing a powerful mechanism for abstraction and dynamic behavior.

**Example**:

 function multiplier (factor) {
   return function(x) {
 return x * factor;
};

By treating functions as first-class citizens, functional programming fosters a coding style that emphasizes composition, reusability, and abstraction, with a clear focus on the behavior of the functions themselves.

Function Composition

Functional composition is a basic concept in functional programming that involves combining multiple functions together to create new functions.

This new function represents the composition of each function, with the output of one function serving as the input of the next function.

Function composition allows you to create complex operations and transformations by breaking them into smaller, reusable, composable parts.

Implementing the compose Function

To harness the power of function composition, we need to define the compose function.

**Example:**

const compose = (…functions) => {
  return (input) => {
   return functions.reduceRight((acc, fn) => {
   return fn(acc);
 }, input);
 }
 };

In this implementation, the compose function accepts any number of functions as arguments using the spread operator …functions.

Use reduceRight to iterate through the functions in reverse order and return a new function that applies each function to the accumulated results.

Conclusion

Functional programming in JavaScript requires you to treat your code as a set of independent, reusable functions that process dat

a without side effects.

Understanding these important concepts will help you write more readable, maintainable, and error-free code.