DEV Community

Cover image for Functors, Monads and Promises in Functional Programming with JavaScript

Posted on

Functors, Monads and Promises in Functional Programming with JavaScript

JavaScript is a versatile high-level programming language commonly used to create interactive elements on web pages. It runs in users' web browsers, enabling dynamic content modification and communication with servers without page reloads. It's widely used in web development and server environments, facilitating the creation of dynamic and interactive web applications.

JavaScript also adapts well to the functional programming style. This is beneficial because it helps developers write clearer and more comprehensible code. By applying functional programming principles in JavaScript, such as avoiding changes to data and using functions for everything, you can create web applications that are efficient and easy to maintain.

Functional programming is a paradigm that treats computations as evaluations of mathematical functions and avoids changing state or using mutable data. This approach is especially useful when working with asynchronous operations or controlled side effects. Concepts such as functors, monads and promises are fundamental in this paradigm, as they facilitate the management of these use cases in a more functional and structured way.


A functor in functional programming is a type of data structure that can be mapped over. This means it provides a map function, allowing you to apply a function to each element it contains without altering the structure itself. In JavaScript, while the term "functor" isn't commonly used, arrays can be seen as functors because they offer a map method. This method applies a transformation to each element, producing a new array with the transformed values.

const names = ["alice", "bob", "charlie", "david"];
const upperCaseNames = => name.toUpperCase());
console.log(upperCaseNames);  // Output: ["ALICE", "BOB", "CHARLIE", "DAVID"]

In this example, the names array is a functor because it is a data structure you can map over with a function. By passing a function to the map method that converts a string to uppercase (name.toUpperCase()), you apply this transformation to each element in the original array without altering it directly. The result, upperCaseNames, is a new array where each string is in uppercase, demonstrating how the map method can efficiently handle transformations in a functional programming style.


Monads are a more advanced concept in functional programming that build upon the principles of functors. They not only apply a function to a value within a context (like functors) but also help in handling operations that involve values wrapped in a context, such as other monads. This capability is especially useful for managing sequences of computations where each step could have complications, like errors or asynchronous results. In JavaScript, promises are a great example of monads. They facilitate chaining asynchronous tasks where each step can return another promise, allowing operations to be linked seamlessly.

const promise = Promise.resolve(10);
const result = promise
    .then(x => {
        return Promise.resolve(x * 2);
    .then(y => {
        console.log(y);  // 20
        return y + 5;
    .then(z => {
        console.log(z);  // 25

In this example, each then() receives the result of a promise, applies a transformation, and returns a new promise. This is characteristic of monads, where the ability to apply transformations and flatten nested structures into a single structure is key.


In JavaScript, promises provide a functional approach to handling asynchronous operations. They represent values that might be available immediately, later, or potentially not at all. The methods then() and catch() are used to manage the flow of these operations and to handle errors in a sequential and organized manner.

    .then(response => response.json())
    .then(data => {
    .catch(error => {
        console.error('Error al obtener los datos:', error);

This example shows how promises allow handling time-dependent operations or other asynchronous processes in a sequential and easy-to-follow manner, in line with the principles of functional programming.

Functors, monads, and promises are powerful concepts in functional programming that make it easy to manage data structures and asynchronous operations effectively. Although these terms are not always used explicitly in JavaScript, the use of arrays and promises demonstrates how these concepts are integrated into everyday language usage, providing programmers with robust tools to handle functional and asynchronous programming more efficiently.

Top comments (0)