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Introduction to Functional Programming (Part 2)

Posted on:March 11, 2023 at 09:00 AM

Introduction to Functional Programming (Part 2)

Functional programming is a programming paradigm that emphasizes immutability, side effect avoidance, and functional composition to create software that is easier to reason about, test, and maintain.

Immutability ensures that once a data value is created, it cannot be modified, thus preventing unexpected changes in the program’s state.

Side effect avoidance minimizes the impact of a function on the overall system state, reducing bugs and making code more predictable.

Functional composition enables building more complex behavior from small, composable functions, increasing modularity and reducing coupling between different parts of the code.

By embracing these concepts, developers can write more reliable and scalable software. Allow me to elaborate further on the three fundamental concepts of functional programming in the following paragraphs.


Immutability is a concept in software engineering that refers to the state of an object or data structure that cannot be modified after it has been created.

Immutable values have several benefits, including:

  1. Thread-safety: Immutable objects are inherently thread-safe because they cannot be modified after they have been created. This eliminates the need for locking mechanisms that can slow down system performance and create concurrency issues.

  2. Improved performance: Immutable objects can be cached and reused because they have a fixed state that does not change. This can reduce the amount of memory allocated and the number of objects created, resulting in improved performance.

  3. Safer concurrency: Immutable objects eliminate race conditions and other concurrency issues that can arise when multiple threads attempt to modify the same object simultaneously.

  4. Predictability: Immutable values are predictable since they cannot be changed once they have been created. This makes it easier to reason about code and reduces the potential for bugs.

In JavaScript, there are several ways to define immutable values. One way is to use primitive data types such as numbers, strings, and booleans, which are immutable by default. Another way is to use object literals or classes that are designed to be immutable.

Here are some examples:

  1. Using const keyword: The const keyword can be used to declare variables with immutable values. Once a variable is assigned a value, it cannot be changed.
const name = "John";

name = "Nikos"; // This will throw: "Uncaught TypeError: Assignment to constant variable."
  1. Object.freeze(): The Object.freeze() method can be used to make an object immutable. Once an object is frozen, its properties cannot be added, deleted, or modified.
const person = Object.freeze({
  name: "John",
  age: 30,

In Typescript, in order to define an immutable property inside an object or a tuple you can use the readonly keyword. Please have in mind that Typescript is a language that compiles to Javascript and the readonly does not actually guarantees you that you value will be immutable. Yes, if you’ll try to mutate the value of a readonly property the compiler will throw an error but do not take that as granted.

Now that we learn about immutability let’s define what a side effect is and learn how to avoid them.

Side effects

Side effects are considered any changes that are made outside the scope of the function or method being executed, including changes to the state of global variables, modifications to data structures, or interactions with external systems.

Side effects can be intentional or unintentional, and they can have both positive and negative consequences. For example, a function that logs an error message when an exception is thrown has a useful side effect that helps with debugging and troubleshooting. However, a function that modifies global variables without properly encapsulating its state can lead to unintended consequences and difficult-to-debug issues.

There are several types of side effects, including:

  1. State modification: Changes to the state of a program or its data structures, such as modifying the values of global variables or updating elements in an array.

  2. I/O operations: Interactions with external systems or devices, such as reading from or writing to a file, making a network request, or printing output to the console.

  3. Exception throwing: Raising an exception or error that interrupts the normal flow of execution and may have side effects such as logging or altering the program state.

Consider the following code:

let count: number = 0;
function increment() {

In this code, the increment() function modifies the value of the count variable, which is outside its scope, creating a side effect. To avoid side effects, we can rewrite the code to return a new value instead:

const count: number = 0;

function increment(currentCount: number) {
  return currentCount + 1;

const incrementedCount = increment(count);

This approach ensures that our program remains predictable and easier to test.

Writing code without side effects has several advantages:

  1. Predictability: Code without side effects is easier to reason about and predict, since its behavior is determined solely by its inputs and outputs. This makes it easier to test and debug.

  2. Reusability: Code without side effects is more modular and can be reused more easily, since it doesn’t rely on external state or variables.

  3. Concurrency: Code without side effects can be safely executed in parallel or distributed environments, since it doesn’t rely on shared state or variables.

  4. Maintainability: Code without side effects is easier to maintain over time, since it doesn’t have hidden or unexpected interactions with other parts of the system.

In functional programming, side effects are typically handled by isolating them into specific sections of the codebase, often called “effectful” functions, and minimizing their impact on the rest of the program.

Here are some common techniques for handling side effects in functional programming:

  1. Pure functions: Writing pure functions that don’t cause side effects is the easiest way to avoid side effects altogether. Pure functions only rely on their inputs and produce a predictable output, making them easy to reason about and test.

  2. Monads: Monads are a design pattern used in functional programming to isolate side effects and encapsulate them in a controlled way. They provide a way to separate the functional, “pure” part of a program from the effectful, “impure” part. We will explore such Monads in other posts so stay tuned!

  3. Higher-order functions: Higher-order functions are functions that take other functions as arguments or return functions as their result. They can be used to encapsulate side effects into smaller, more manageable functions.

  4. Functional reactive programming (FRP): FRP is a programming paradigm that focuses on handling side effects in a declarative way. It provides a way to model changes over time using streams or observables, and allows side effects to be isolated into specific “sinks” or “reducers”.

Now that we defined side-effects and pure functions lets continue with the functional composition

Functional Composition

Functional composition is a technique used in functional programming to combine multiple functions together to create more complex functionality. It involves taking the output of one function and using it as the input to another function, in a chain or pipeline.

Consider the following example:

function double(num: number): number {
  return num * 2;

function addFive(num: number): number {
  return num + 5;

function subtractTwo(num: number): number {
  return num - 2;

const doubleAndAddFiveAndSubtractTwo = (num: number) =>

console.log(doubleAndAddFiveAndSubtractTwo(3)); // (((3 * 2) + 5) - 2) Output: 9

Here, the doubleAndAddFiveAndSubtractTwo function is created by composing the double, addFive and subtractTwo functions together, using the output of double as the input to addFive and the output of addFive as the input to subtractTwo. This creates a new function that doubles a given number, then adds 5 and subtracts 2 to the result, in a single step.

Functional composition is a powerful technique that allows us to create complex functionality by combining simpler building blocks, without introducing side effects or modifying external state.

But this looks a bit complicated? Is this the only way to compose functions in javascript?

Fp-ts to the rescue

Fortunately for us, Giulio Canti has created an excellent library called fp-ts which aims to allow developers to use popular patterns and abstractions that are available in most functional languages, in Typescript. We will use extensively the fp-ts library in the following posts when we will explain some of the core concepts of the Category Theory. Now lets use the pipe function that fp-ts provides us with in order to write a more readable and easy-to-maintain program.

import { pipe, flow } from "fp-ts/functions";

function double(num: number): number {
  return num * 2;

function addFive(num: number): number {
  return num + 5;

function subtractTwo(num: number): number {
  return num - 2;

// Example of functional composition using `pipe`
console.log(pipe(3, double, addFive, subtractTwo)); // Outputs: 9

// Another example of functional composition using `flow`
const doubleAndAddFiveAndSubtractTwo = flow(double, addFive, subtractTwo);
console.log(doubleAndAddFiveAndSubtractTwo(3)); // Outputs: 9

See? It’s really fun to apply functional composition using the pipe (or flow) function.

Unfortunately, Javascript, unlike other Functional Programming languages like Haskell, Scala, F# etc., does not have a pipe operator implemented in its core. There is a great proposal for adding a useful pipe operator to JavaScript but still we have a lot of work to do to get there.


In summary, we have gained an understanding of immutability and its benefits in our code. Additionally, we have learned how to write side-effect-free code through the use of pure functions, and we have also explored the concept of functional composition and its practical application in TypeScript. I hope this has been an enlightening and enjoyable learning experience for you. See you in the next posts.