Functional Programming in JavaScript

Bharath Kumar Murugan
3 min readMay 8, 2023

Functional Programming is a programming paradigm that emphasizes the use of pure functions to solve problems. It is a popular programming paradigm in modern software development, and JavaScript is no exception. In this blog post, we will explore Functional Programming in JavaScript in simple English, along with some examples.

What is Functional Programming?

Functional Programming is a programming paradigm that treats computation as the evaluation of mathematical functions. The core idea of Functional Programming is to write code that is modular, maintainable, and testable, by using pure functions. A pure function is a function that always returns the same output for the same input and has no side effects. That means it does not change any state outside of its scope.

Example of a Pure Function:

function add(a, b) {
return a + b;
}

In the above code, the add function is a pure function because it always returns the same output for the same input and does not have any side effects.

Functional Programming Concepts in JavaScript

  1. First-Class Functions:

Functions in JavaScript are first-class citizens, which means they can be treated like any other data type. Functions can be assigned to variables, passed as arguments to other functions, and returned as values from functions.

Example of a First-Class Function:

const multiply = function(a, b) {
return a * b;
};

const result = multiply(5, 10);
console.log(result); // Output: 50

2. Higher-Order Functions:

A higher-order function is a function that takes another function as an argument or returns a function as its result. Higher-order functions are an essential concept in Functional Programming because they allow us to write code that is more modular and composable.

Example of a Higher-Order Function:

const add = function(a, b) {
return a + b;
};

const multiply = function(a, b) {
return a * b;
};
const calculate = function(func, a, b) {
return func(a, b);
};
const result1 = calculate(add, 5, 10);
console.log(result1); // Output: 15

const result2 = calculate(multiply, 5, 10);
console.log(result2); // Output: 50

In the above code, the calculate function is a higher-order function because it takes another function as an argument.

3. Pure Functions:

As mentioned earlier, pure functions are functions that always return the same output for the same input and have no side effects. Pure functions are easier to reason about and test, and they help to reduce bugs in our code.

Example of a Pure Function:

const add = function(a, b) {
return a + b;
};

In the above code, the add function is a pure function because it always returns the same output for the same input and has no side effects.

4. Immutability:

In Functional Programming, immutability is the concept of not changing the state of an object. Instead of changing the state of an object, we create a new object with the updated state.

Example of Immutability:

const array1 = [1, 2, 3];
const array2 = array1.concat(4);

console.log(array1); // Output: [1, 2, 3]
console.log(array2); // Output: [1, 2, 3, 4]

In the above code, the concat method creates a new array with the updated state instead of changing the state of the original array.

Functional Programming is a powerful programming paradigm that can make our code more modular, maintainable, and testable.

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Bharath Kumar Murugan
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FrontEnd Developer | Javascript Enthusiast | Linux | Docker