My journey to understanding circular dependencies began with a casual code change. I was working on a feature where most of my changes involved two services — let’s call them UserService and ProfileService. At one point, I already had a userId and just needed to fetch the related profile. Instead of writing something new, I checked the ProfileService file, and luckily, there was already a neat little getProfileById function. Great, I used it, and the task moved forward smoothly.

But then, while scrolling through the same file, I noticed something that made me pause. Some of the functions inside ProfileService were already reaching back into UserService.

My quick fix had unknowingly created a situation where both services depended on each other. That was the moment I realised I had brushed up against a classic circular dependency, not an obvious bug, but definitely a design smell.

Nothing broke immediately, the code still ran fine. But it didn’t sit right with me. What if I added one more call the other way? What if someone else on the team, unaware of this hidden relationship, made changes in one service without realising its impact on the other? The more I thought about it, the more it felt like a trap waiting to spring.

At its core, a circular dependency happens when two or more modules, classes, or even functions rely on each other to function. It’s like a snake eating its own tail. Neither can fully exist without the other, leading to a standstill.

How Circular Dependencies Creep In

The tricky part about circular dependencies is that they don’t always appear in one big obvious jump. They usually sneak in gradually, through small, reasonable-looking decisions. That’s exactly what happened in my case. I only needed one function from ProfileService. Later, someone else might need one from UserService. Step by step, the two files get tangled.

Here’s a simplified version of how it happens:

// userService.js
import { getProfileById } from "./profileService.js";
export function getUserById(userId) {
 return { id: userId, name: "Alice" };
}
export function getUserWithProfile(userId) {
 const user = getUserById(userId);
 const profile = getProfileById(userId); // <-- getProfileById imports userService, completing the cycle
 return { ...user, profile };
}

// profileService.js
import { getUserById } from "./userService.js";
export function getProfileById(profileId) {
 const user = getUserById(profileId); // This triggers the cycle!
 return { id: profileId, bio: "Loves coding", user };
}

// Running this setup may cause errors like:
// TypeError: getUserById is not a function if
// Node.js cannot resolve the exports before
// one file tries to use the other's function.

At first, both files make sense on their own. But if you look closer:

• userService.js imports profileService.js.
• profileService.js also imports userService.js.

That’s a circular dependency. Each service depends on the other, creating a cycle.

💡 Notice how natural this looks? You’re just trying to avoid duplicate logic and reuse what’s already there. But suddenly, the design has become fragile.

What Does a Circular Dependency Look Like?

While the application might sometimes run, in many cases this setup will cause errors at runtime. For example, Node.js may throw TypeError: getUserById is not a function because the dependent module’s exports are incomplete at the time they're imported.

At first glance, it doesn’t look like a problem. The application runs fine, the data flows as expected, and you don’t notice any errors. But the risk is lurking beneath the surface:

• Tight Coupling → The two services are now locked together. Any change in one may ripple unexpectedly into the other.
• Initialisation Issues → In some languages, a circular dependency can cause a module to load only partially, leading to runtime errors that are hard to trace.
• Reduced Reusability: When modules are so intertwined, it becomes difficult to reuse one module independently in another project. You end up having to drag along unnecessary baggage.
• Harder Testing → It becomes difficult to isolate one service in unit tests because it drags the other one along.
• Fragility Over Time → As the codebase grows, the cycle becomes harder to untangle and makes refactoring painful.

That’s why circular dependency is considered an architectural code smell. It might not bring your system crashing down on day one, but it leaves behind a fragile foundation that’s easy to trip over later.

How to Break the Circle

// mediator.js
import { getUserById } from "./userService.js";
import { getProfileById } from "./profileService.js";
export function getUserWithProfile(userId) {
 const user = getUserById(userId);
 const profile = getProfileById(userId);
 return { ...user, profile };
}

// Then, neither service imports the other directly,
// and both can remain independent for testing and reuse.

Breaking circular dependencies often involves rethinking your design and applying solid software engineering principles. Here are some common strategies:

Introduce an Interface or Abstraction

This is often the most elegant solution. If Module A needs a specific behavior from Module B, and Module B needs a specific behavior from Module A, define an interface for that behavior. Both modules can then depend on the interface, and concrete implementations can be provided elsewhere. For example, instead of a User class depending on an Profile class and vice-versa, both might depend on an IUserNotifier interface and an IProfileProcessor interface, respectively.

Adopt an Event-Driven Architecture

Instead of direct calls, modules can communicate by publishing and subscribing to events. Module A can emit an event when something happens, and Module B can listen for that event, and vice-versa. This decouples the modules significantly.

Use the Dependency Inversion Principle (DIP)

A core SOLID principle, DIP states that “high-level modules should not depend on low-level modules. Both should depend on abstractions.” By pushing dependencies toward abstractions, you naturally break circularity.

Introduce a New Module (Mediator/Orchestrator)

Sometimes, the circularity arises because two modules are trying to do too much. A new, higher-level module can be introduced to mediate the communication between the original two, taking on the responsibility of coordinating their interactions. This mediator then depends on both, but they don’t depend on each other.

Refactor and Rescope

Sometimes, the simplest solution is to refactor your code. Perhaps the functionality causing the circularity belongs in a different module, or the responsibilities of the existing modules need to be re-evaluated and separated more cleanly.

Pass Dependencies as Arguments

In some cases, especially with functions, you can break a circular dependency by passing the required dependency as an argument rather than relying on a direct import or global access.

Prevention is Key

While there are strategies to fix circular dependencies, the best approach is to prevent them in the first place. Adopting good design principles like the Single Responsibility Principle (SRP) and the Dependency Inversion Principle from the outset can significantly reduce the likelihood of encountering these issues.

Additionally, tools for static analysis can help identify circular dependencies early in the development cycle, allowing you to address them before they become deeply ingrained in your codebase.

Conclusion

Circular dependencies are a common pitfall that can lead to brittle, hard-to-maintain code. By understanding their causes and applying appropriate design patterns and principles, you can effectively break these vicious circles and build more robust, flexible, and understandable software systems.

Next Steps

Circular dependencies are just one type of “code smell” that can lead to a fragile codebase. The journey from a good programmer to a great one often involves learning to recognise these design flaws early and addressing them proactively.

To help you on your journey, here is a more comprehensive list of common code smells. Use this as a checklist or a guide to reflect on your own code.

A Quick Guide to Common Code Smells

What’s a code smell you’ve encountered that surprised you the most? Share your stories in the comments below, Happy coding!

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