Code Smell 209 - Side Effects

Photo by Daan Mooij on Unsplash

Code Smell 209 - Side Effects

Global scope is easy or a nightmare, or both

TL;DR: Avoid side effects on your code.


  • Coupling

  • Least Astonishment Principle violation


  1. Favor referential transparency


Referential transparency always produces the same output for a given input and does not have any side effects, such as modifying global variables or performing I/O operations.

A function or expression is referentially transparent if it can be replaced with its evaluated result without changing the behavior of the program.

This property allows for easier reasoning about the behavior of a program and enables optimizations such as caching and memorization.

Functions are treated as mathematical expressions that map inputs to outputs.

Sample Code


let counter = 0;

function incrementCounter(value: number): void {
  // Two side effects
  counter += value; // it modifies the global variable counter 
  console.log(`Counter is now ${counter}`); // it logs a message to the console.


let counter = 0;

function incrementCounter(counter: number, value: number): number {  
  return counter + value; // Not too efficient  


[X] Automatic

Most linterns can warn you when accessing the global state or Functions and create side effects.


  • Global


Functional Programming is amazing and can teach us a lot about how to write clean code.

We need to understand its pillars.



Code Smells are my opinion.


Photo by Daan Mooij on Unsplash

The most effective debugging tool is still careful thought, coupled with judiciously placed print statements.

Brian W. Kernighan

This article is part of the CodeSmell Series.