Anti-patterns to avoid in JavaScript

Anton Ioffe - September 25th 2023 - 18 minutes read

Welcome to this detailed probe into the elusive world of JavaScript anti-patterns. As developers, we are continually searching for ways to refine our code, identify bottlenecks, and streamline our workflows. This scrutiny often revolves around the improvement of existing, functional code. But what about the darker side of JavaScript programming? Those all-too-common practices that, while they may not outright break your code, can muddle its maintainability, degrade performance, and complicate your life as a developer? Unfortunately, those paths are more often than we'd like to admit - and they're what we refer to as anti-patterns.

By diving into this comprehensive guide, you'll arm yourself with the knowledge required to identify and mitigate these misleading programming paths in your JavaScript coding universe. We'll roam the plains of common JavaScript anti-patterns, dive deep into JavaScript-specific pit-falls, and unmask the effects they exude on your applications. We'll whisk through comparisons between innocuous design patterns and their anti-pattern counterparts, expose prominent anti-patterns in server-side JavaScript, notably in Node.js, and finally, arm you with foolproof strategies to circumvent these traps.

So, prepare for an enlightening journey that will transform your perspective, refine your skills, and fundamentally, make you a better JavaScript developer. This is not just about learning what to do, but more importantly— learning what not to do. Let's lean into the dark side of JavaScript—to better illuminate our path towards well-structured, efficient, and high-performing JavaScript code.

Deciphering Anti-Patterns in Modern JavaScript Programming

In the vast realm of JavaScript programming, the concept of anti-patterns constantly lurks behind the lines of code written by developers. Understanding these anti-patterns is quintessential to writing efficient, maintainable, and optimized code.

What are Anti-patterns?

Anti-patterns are practices that may initially appear beneficial, but in the longer run, they cause more harm than good. They can result in unoptimized or unmaintainable code, leading to difficulty in debugging, and lost development time which could have been better utilized improving features or designing superior architecture.

The Problem with Anti-patterns

The main problem anti-patterns pose is their duplicitous nature. They aren’t blatant errors in the code; instead, they are often masked as seemingly valid solutions to recurring problems. However, they introduce subtle inefficiencies and complexities that can ambush unwary developers.

Let's understand this with an example:

function getHighestValue(array){
    array.sort();  // Sorting the array
    return array[array.length - 1];  // Returning last element i.e., highest value

In the above code, the developer sorts the array first and then returns the highest value. Although this works correctly, it incurs unnecessarily high computational cost. The .sort() method implements a comparison sort that's not the optimal solution. A simple traversal to find the maximum value is far more efficient.

The correct code should be:

function getHighestValue(array){
    return Math.max(...array);  // Efficient way of finding maximum value

Avoiding Anti-Patterns: A Necessity for Optimal Code

Incorporating anti-patterns into your JavaScript code poses significant challenges in both software development and maintenance. Removing these patterns is essential for several reasons:

  • Performance: Anti-patterns often result in slower code execution and higher memory consumption. Efficient code is particularly critical for JavaScript because it's executed client-side and affects user experience directly.
  • Maintainability: Anti-patterns increase code complexity which in turn leads to higher maintenance costs. This impacts the agility of a team in terms of implementing new features and fixing bugs.
  • Readability: Unintuitive code that follows anti-patterns is more difficult to read and understand. Imagine being a new developer on a team trying to understand code plagued with anti-patterns!


In conclusion, it is critical to understand and avoid anti-patterns in JavaScript. Doing so vastly improves the performance of the code, reduces maintenance overhead, increases readability, and ultimately leads to smoother development cycles. In the next sections, we will dive into the details of some specific JavaScript anti-patterns and discuss how to avoid them.

Questions to ponder on:

  • Can you identify an anti-pattern in a piece of JavaScript code you've written recently?
  • What changes could you make to refactor that piece of code and avoid the anti-pattern?
  • Are there anti-patterns specific to JavaScript that you frequently encounter? Can you think of ways to avoid them?

Common JavaScript Anti-Patterns: A Comprehensive Catalogue

In JavaScript development, it's important to be aware of certain anti-patterns that can detrimentally affect your code's performance, readability, and maintainability. In this section, we'll focus on some of the common JavaScript anti-patterns, providing real-world code examples to help you better understand and avoid these pitfalls in your own work.

Global Namespace Pollution

It's easy to pollute the global namespace in JavaScript, particularly if you're not using a module system. While this may seem harmless, it can lead to unexpected consequences when different parts of your code unintentionally interact or overwrite each other's variables.

// Bad practice
var globalVar = 'Hello, glob!';

function myFunction() {
    globalVar = 'Hello, World!';

myFunction(); // Outputs: 'Hello, World!'
console.log(globalVar); // Outputs: 'Hello, World!'

Here's a better approach, encapsulating the variable within the function:

// Good practice
function myFunction() {
    var localVar = 'Hello, World!';

myFunction(); // Outputs: 'Hello, World!'
console.log(localVar); // Uncaught ReferenceError: localVar is not defined

By limiting the exposure of our variables, we avoid accidental overwrites and reuse, and thereby keep our global namespace clean.

Improper Use of Truthy and Falsy Values

In JavaScript, all values have an inherent boolean-truthy or falsy-ness. A common mistake is to rely on this feature without understanding it fully, leading to potentially confusing or incorrect results.

// Bad practice
const myArr = [0, 1, 2, 3];
const result = myArr.find(el => el); 
console.log(result); // Outputs: 1

In the above code, the find method returns the first truthy element in the array, skipping 0 because it's falsy in JavaScript.

A better approach? Be explicit about what you're looking for:

// Good practice
const myArr = [0, 1, 2, 3];
const result = myArr.find(el => el !== undefined);
console.log(result); // Outputs: 0

Here, we're specifying that we want the first element that isn't undefined.

Modifying the DOM in a Loop

Manipulating the DOM is expensive in terms of performance. One common anti-pattern is to modify the DOM inside a loop, which can lead to inefficient reflows and repaints.

// Bad practice
for(let i = 0; i < 1000; i++) {
    const div = document.createElement('div');
    div.textContent = `Div number ${i}`;

A better way to handle repetitive DOM manipulation is to use a Document Fragment, which allows you to collate all your changes and append them in one go:

// Good practice
const fragment = document.createDocumentFragment();

for(let i = 0; i < 1000; i++) {
    const div = document.createElement('div');
    div.textContent = `Div number ${i}`;


Each of these anti-patterns presents a unique set of challenges and nuances, making it crucial to have a thorough understanding before attempting to diagnose and rectify mistakes. Can you think of some other scenarios where these issues might arise? What measures can you take as a developer to be more mindful of these anti-patterns? These thoughts could lead to more efficient and higher-quality code.

JavaScript's Unique Pitfalls: Anti-Patterns Specific to JavaScript

One of the unique aspects of JavaScript is its flexibility, which also proves to be a double-edged sword. It provides developers the liberty to adopt different programming styles, leading to several anti-patterns emerging, primarily due to misinterpretation and misuse of its complex features. Below are a few JavaScript-specific anti-patterns that you may encounter in your coding journey, complete with real-world code examples and solutions.

Misuse of Constructors

JavaScript constructors are essential for object creation. However, they can easily lead to some undesirable results, especially when misused.

function Bus(model, year) {
    this.model = model;
    this.year = year;

const miniBus = Bus('Mini', 2020); // Wrong way of invoking a constructor
console.log(miniBus); // Outputs "undefined"

As you can see, invoking the constructor without the new keyword leads to an undefined result because this points to the global object. By forgetting new, you're essentially corrupting the global namespace. The constructor function should have been invoked as follows:

const miniBus = new Bus('Mini', 2020);  // Correct way of invoking a constructor
console.log(miniBus); // Outputs "Bus {model: 'Mini', year: 2020}"

Hoisting Gotchas

JavaScript's hoisting behavior can lead to unexpected results, especially when it comes to function and variable declarations.

console.log(hoistedVar); // Outputs "undefined" but no ReferenceError is thrown
var hoistedVar = 'hoist me up!';

Here, even though the variable hoistedVar is not defined until after the log statement, JavaScript's hoisting feature moves the variable declaration (but not the initialization) to the top of its scope.

This behavior could lead to confusion and unexpected results. Thus, it's recommended to always declare variables at the top of their scope to make the code more predictable and easier to read.

Misuse of eval()

The eval() function in JavaScript is a powerful tool that parses and executes string-based JavaScript code. Its misuse can lead to numerous issues including performance pitfalls and security vulnerabilities.

const data = 'console.log("Hello, World!")';
eval(data); // Outputs "Hello, World!"

Here, eval() is used to execute JavaScript code stored as a string, unnecessarily adding a layer of interpretation and opening the door to potential script injection attacks. If possible, try to avoid eval(). If you need to handle dynamically generated code, consider safer alternatives like Function constructor or setTimeout.

Remember, understanding a language's idiosyncrasies and avoiding pitfalls can greatly improve your programming skills and the quality of your code. Questions for reflection: How can you refactor your existing JavaScript code to avoid these common anti-patterns? Do you know any other JavaScript-specific pitfalls that were not mentioned here? What are the common preventive measures you follow to avoid getting caught in these pitfalls? Reflect on these to continue growing as an efficient JavaScript developer.

The Impact of Anti-Patterns on Application Performance and Code Readability

Anti-patterns in JavaScript can degrade the performance of an application and decrease code readability significantly. By understanding the adverse effects of these poor practices, we can avoid them and write cleaner, more efficient code.


The primary way anti-patterns impact application performance is by consuming more memory than necessary or leading to inefficient CPU usage. For instance, consider the following code snippet:

function calculateSum(arr){
    let sum = 0;
    for(let i=0; i<1000000; i++){
        sum = sum + arr[i];
    return sum;

This function will add up the first million items of arr, even if the array does not contain that many elements. This kind of processing not only wastes CPU cycles but can also lead to erroneous results. When writing code, always ensure no unnecessary processing is occurring.

Code Complexity and Readability

Another significant impact of JavaScript anti-patterns is on code complexity and code readability. Anti-patterns often involve writing more complex and convoluted code than necessary, making the code more difficult to understand, troubleshoot, and maintain.

Consider a case where developers have used nested ternary operators:

let result = condition1 ? value1 : condition2 ? value2 : condition3 ? value3 : default_value;

While this may look clever and efficient at first sight, it’s a classic example of an anti-pattern that increases code complexity and decreases readability. Instead, using if...else statements or a switch-case block can vastly improve code readability.

Modularity, Reusability, and Best Practices

Now let's consider a code which doesn't apply the principles of modularity and reusability. Modularity enables you to write separate, standalone pieces of code (or modules) that can be reused across your codebase. Writing code that is not modular or reusable is more time-consuming, and more prone to errors and bugs.

An example of this might be a function which fulfills multiple tasks rather than splitting them into modular pieces.

function processUser(user){
    // calculating age
    let age = calculateAge(user.birthdate);
    // validating user data
    let isValid = validateUserData(user);
    // sending email

While this function is very convenient, it would be much more maintainable and reusable if the calculation of the age, the validation, and the sending of the email were separated into different functions.


Security is often overlooked when discussing anti-patterns. However, neglecting security best practices can lead to risky anti-patterns. A common one is not validating or sanitizing user input on the server-side.

Consider this portion of an insecure Node.js Express application:'/addUser', function (req, res) {
  var user = req.body;
  // ... add user directly to the database without validating

In this case, a user can send harmful data that can compromise the database or the server. Dealing with user input should always include validation, and where necessary, sanitization.

While the merits and drawbacks of different approaches in JavaScript development is a heated topic, there’s universal agreement that developers should steer clear of known anti-patterns. They impact the overall performance of a JavaScript application and make your code more complex and harder to read than it needs to be. Do you review your code from time to time to ensure you're not falling for these common traps?

Comparative Analysis: Anti-patterns vs. Design Patterns

In software development, design patterns are recurring solutions that solve common design problems in software design. On the other hand, anti-patterns are commonly repeated but poor practices that initially appear beneficial but result in bad outcomes. Let's explore some common design and anti-patterns in JavaScript, focusing on Singleton and Factory patterns.

Singleton Pattern vs. Singleton Anti-pattern

The Singleton pattern restricts the Instantiation of a class to a single instance. It's a useful option for coordinating actions across the system, creating a global point of access, and preserving from discrepancies by repeated instances.

In JavaScript, a Singleton pattern implementation might look like:

let Singleton = (function () {
    let instance;
    function createInstance() {
        let object = new Object('I am the instance');
        return object;
    return {
        getInstance: function () {
            if (!instance) {
                instance = createInstance();
            return instance;
let instance1 = Singleton.getInstance();
let instance2 = Singleton.getInstance();

console.log(instance1 === instance2);  // true, it's the same instance

On the other hand, excessive use of the Singleton pattern can lead to an anti-pattern where it becomes a global variable and creates tight coupling or hides dependencies in your code. Singleton objects can carry a state which might lead to unexpected behavior when changes are made.

Factory Pattern vs. Factory Anti-pattern

Factory patterns are creational patterns that provide an interface for creating objects in a superclass, but allows subclasses to alter the type of objects that will be created. They are particularly useful when you want to create an object without specifying the class explicitly.

In JavaScript, a Factory pattern implementation might look like:

function CarMaker() { } = function() { 
    return `I have ${this.doors} doors`

CarMaker.factory = function(type) {
    let newcar;
    if (type === "Sportscar") {
        newcar = new Sportscar();
    } else if (type === "SUV") {
        newcar = new SUV();
    } else {
        newcar = new CarMaker();

    return newcar;

While a Factory pattern is great at hiding complexity, it potentially leads to an anti-pattern when you start creating factories for every object in your setup. It might make the code unnecessarily complex and more challenging to debug. The dependency issued by factories might also be hidden, making the code harder to understand.

Therefore, it is crucial to understand the circumstances under which these patterns are applicable and consider possible trade-offs. Pattern use should enhance the code structure and not complicate it. Always remember, anti-patterns might look great initially, but they bring complexity and potential issues in the long run.

Are there evident anti-patterns you have noticed in your JavaScript code that you weren't aware of, and how do they affect your code's performance, maintainability, and readability? How do you plan on refactoring those areas to adhere more to best practices?

Anti-patterns in Server-side JavaScript: A Closer Look at Node.js

In the realm of server-side JavaScript development, a popular choice among developers is Node.js. Node.js provides a rich environment for the development of efficient web servers and networking applications, thanks to its event-driven architecture and asynchronous I/O. However, its unique strengths also introduce specific pitfalls or anti-patterns which can lead to bloated, unoptimized, and error-prone code if not correctly managed. In this section, we will elaborate on these anti-patterns with a particular emphasis on blocking I/O operations and incorrect async handling.

Blocking I/O Operations

One of the core principles of Node.js is its non-blocking I/O model. Leveraging the power of asynchronous events, it can handle many simultaneous client connections efficiently. Ironically, one of the most common anti-patterns developers fall into is writing code that inadvertently blocks I/O, hamstringing this efficiency and often leading to sharp performance drops.

Take file reading, for example. Using the synchronous readFileSync() function:

const fs = require('fs');
let data = fs.readFileSync('/path/to/file');

This is a blocking operation: the server won’t do anything else until it finishes reading the file. This monolithic approach creates a bottleneck and severely undermines the concurrent processing capabilities of Node.js.

The correct pattern would be to use the async version of the same function:

const fs = require('fs');
fs.readFile('/path/to/file', (err, data) => {
  if (err) throw err;

In this way, other operations can continue to process in parallel while the file is being read, maintaining the non-blocking nature of Node.js.

Incorrect Async Handling

Another common pitfall arises when you attempt to treat asynchronous code as if it were synchronous. This usually occurs when developers are not entirely comfortable with the async/await model, or when they come from a synchronous-language background.

Consider the following example:

for (let i = 0; i < myArray.length; i++) {
  setTimeout(() => {
    console.log(`Index: ${i}`);
  }, 1000);

You might expect the loop to block iteration for 1 second on each cycle. In reality, it doesn't. This is because setTimeout() is an asynchronous function that will execute its callback after a designated time, but will not block the rest of the code from running.

A more optimal approach would involve using async/await to correctly handle asynchronous operations:

const delay = ms => new Promise(resolve => setTimeout(resolve, ms));

(async () => {
  for (let i = 0; i < myArray.length; i++) {
    await delay(1000);
    console.log(`Index: ${i}`);

Nested Callbacks and Callback Hell

When working with asynchronous code, it's easy to fall into the trap of creating nested callbacks – also known as 'callback hell'. This occurs when you have asynchronous operations depending on the results of each other, resulting in multiple levels of callbacks. The code can quickly become unreadable and hard to maintain.

    getMoreData(a, function(b){
        getMoreData(b, function(c){
            getMoreData(c, function(d){
                // final callback

An effective way to manage these nested callbacks is by using Promises, or async/await:

async function getAllData() {
  const a = await getData();
  const b = await getMoreData(a);
  const c = await getMoreData(b);
  const d = await getMoreData(c);
  // continue processing

getAllData().catch(error => console.error(error));

Note that error handling becomes significantly more straightforward with the use of async/await and promises.

The challenge and beauty of JavaScript lie in its asynchronous nature. As developers, our task is to harness this power while avoiding falling into these common pitfalls. So, while weaving the intricate web of a Node.js application, you must be mindful of these anti-patterns for efficient, bloat-free, and error-resilient code. Keep these bad practices in mind, review your codebase from time to time, and enjoy your Node.js journey!

Avoiding the Traps: Strategies for Preventing Anti-patterns and Writing Cleaner Code

Strategic Prevention of Anti-Patterns

JavaScript, spanning across various environments from the browser to the server-side with Node.js, is one of the most popular programming languages in the web development landscape. However, power comes with responsibility. And, as any seasoned JavaScript developer knows, the language's dynamic nature and flexibility can often be a double-edged sword, leading to various anti-patterns, if not utilized correctly.

The following strategies are based on best practices adopted by experienced JavaScript developers to help you proactively avoid anti-patterns and write cleaner, more efficient code.

Embrace the Principle of Least Privilege

One of the core principles in secure coding is the Principle of Least Privilege (PoLP), which states that a user should be given the least set of permissions necessary to perform their tasks. It applies to JavaScript as well. When defined, variables should be limited in scope as much as possible, avoiding global scope to prevent name collisions and unwanted side effects.

// Global Scope - avoid this
var myName = 'John Doe';

// Local Scope - preferred
function displayName(){
    var myName = 'John Doe';

In the example, the myName variable is contained within the displayName() function, preventing possible name collisions and limiting the scope where it can be modified.

Minimize the Use of this

JavaScipt's this keyword is often misunderstood and can lead to some unexpected behavior, mainly when used in different contexts. It's better to minimize its use. Reserve it for situations where the context is straightforward and can't lead to bugs, for example in object constructors, and methods in object-oriented JavaScript.

Make Use of Strict Mode

JavaScript's "use strict" directive enables strict mode, which can help spot common coding mistakes such as using a variable before declaring it. Strict mode makes it easier to write "secure" JavaScript by notifying you about potential issues earlier rather than failing silently.

'use strict';

x = 3.14; // This will cause an error as x is not defined

Avoid Memory Leaks

It's good practice to be careful with objects, closures, and globals to prevent memory leaks. Be mindful when assigning a value to a global variable or to a property of an object - you might unintentionally increase the memory consumption of your application. Make sure to remove event listeners and dom references when no longer needed, and to keep the number of closures and globals to a minimum.

// Potential memory leak situation
var theThing = null;
function replaceThing() {
    var originalThing = theThing;
    var unused = function () {
        if (originalThing) {
    theThing = {
        someMethod: function () {
setInterval(replaceThing, 1000);

In the code above, the function unused forms a closure, which refers to originalThing, leading to an unintended memory leak. This situation can be avoided by making sure that unused does not have a reference to originalThing.

Error Handling Best Practices

Proper error handling is crucial for any software, especially in JavaScript, where so many things can go not as expected. You should make conscientious use of try/catch blocks, handle your asynchronous code errors correctly, and throw your own errors when necessary to alert yourself to problems you might not be aware of.

In conclusion, by embracing best programming practices and recognizing the traps of common anti-patterns in JavaScript, we can write code that is cleaner, easier to understand, and maintains the overall performance and consistency of our application. Each of these strategies promotes a more instinctive coding style and steers us away from the common pitfalls seen in JavaScript codebases. We are, after all, not just writing code for machines, but also for other developers who might need to maintain, update or refactor our code in the future.


In this article about anti-patterns to avoid in JavaScript, the author discusses the common practices that can lead to code that is hard to maintain, performs poorly, and is difficult to read. They explain that anti-patterns are practices that may initially seem beneficial but end up causing more harm than good in the long run. The article covers various examples of anti-patterns in JavaScript, such as global namespace pollution, improper use of truthy and falsy values, modifying the DOM in a loop, and more. The author emphasizes the importance of avoiding anti-patterns in order to improve code performance, maintainability, and readability.

The key takeaways from this article are the understanding of what anti-patterns are and the problems they can cause, the knowledge of common JavaScript anti-patterns and how to avoid them, and the awareness of anti-patterns specific to JavaScript and Node.js. The article encourages readers to reflect on their own code and identify any anti-patterns they may have unknowingly implemented, and also provides questions for readers to consider for further self-reflection and improvement.

A challenging technical task related to the topic of anti-patterns in JavaScript could be to review a codebase or a specific section of code and identify any anti-patterns present. The reader could then refactor the code to eliminate these anti-patterns and improve the code's performance, maintainability, and readability. Additionally, the reader could research and identify any other JavaScript-specific anti-patterns that were not covered in the article and propose ways to avoid them. This task would require a deep understanding of JavaScript best practices, common anti-patterns, and the ability to critically analyze and refactor code.

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