As your full stack application grows, so does the complexity of the code. You start adding more features, more files, and more folders. Suddenly, your codebase is deeply nested and full of modules that depend on each other. This is when a common but confusing problem can show up: circular dependencies.
A circular dependency happens when two or more modules depend on each other in a loop. For example, Module A uses something from Module B, but Module B also uses something from Module A. This can create problems in your application like broken imports, undefined variables, or unexpected behavior at runtime.
Circular dependencies are often hard to spot and fix, especially in large projects. For those taking full-stack developer classes, understanding how circular dependencies work and how to avoid them is an important step in becoming a professional developer.
In this blog, we’ll explain what circular dependencies are, how they affect full stack applications, how to detect them, and most importantly, how to fix or prevent them in the first place.
What Are Circular Dependencies?
Let’s start with a simple example:
// fileA.js
import { functionB } from ‘./fileB.js’;
export function functionA() {
functionB();
}
// fileB.js
import { functionA } from ‘./fileA.js’;
export function functionB() {
functionA();
}
In this code, fileA.js imports from fileB.js, and fileB.js imports from fileA.js. This creates a loop. When JavaScript tries to load these files, it doesn’t know which one to finish first. This can result in undefined errors or cause your app to crash.
These issues become harder to manage in larger projects where files are deeply nested in folders and many modules depend on each other.
Why Circular Dependencies Are a Problem in Full Stack Development
In full stack applications, you often split your code into frontend and backend. Each side may have its own folder structure, but they often share logic, such as models, validation, or utility functions. When these files reference each other without clear planning, circular dependencies can sneak in.
Problems caused by circular dependencies include:
- Runtime Errors: Functions or objects may be undefined or not work as expected.
- Slower Load Times: Your app may take longer to start because the system is resolving dependencies in loops.
- Testing Issues: Unit tests may fail or behave strangely.
- Harder Debugging: It’s not always clear where the circular dependency started.
- Code Confusion: Developers may struggle to understand which module does what.
These problems grow quickly in projects with deeply nested folders, especially if you’re not careful about file imports.
How to Detect Circular Dependencies
Sometimes you may see errors like:
TypeError: Cannot read property ‘something’ of undefined
This can be a sign of a circular dependency. But it’s better to use tools that can automatically detect them for you:
- madge: A popular tool that creates a visual map of your project’s dependencies.
- Install: npm install -g madge
- Run: madge –circular src/
- dependency-cruiser: A more advanced tool for analyzing and fixing dependencies.
Using these tools regularly helps you catch circular dependencies before they cause real damage.
Common Causes of Circular Dependencies
Here are a few ways circular dependencies usually happen:
1. Bi-directional Imports
This is when two files import from each other. Example: user.js imports auth.js, and auth.js also imports user.js.
2. Shared Constants or Models
You create a file for shared constants or data models, but then import things into that file from modules that already use those constants. This can create a loop.
3. Utility Functions with Imports
Sometimes utility files, which should be clean and independent, start importing other logic-heavy files, leading to unexpected loops.
4. Deeply Nested Folder Structures
Long paths like ../../../../models/user.js make it harder to trace dependencies. As the number of imports grows, so does the chance of creating a circular reference.
If you’re studying through a full stack course, your instructor may recommend keeping utility and shared files flat and simple to avoid these issues.
How to Fix Circular Dependencies
Fixing circular dependencies requires changes in how your project is structured and how you think about module relationships.
1. Refactor Code to Remove Bi-directional Imports
Look at the two files that depend on each other. Can you move shared functions into a third file that both can import?
Example:
// sharedFunctions.js
export function commonLogic() {
// shared code
}
Now, both fileA.js and fileB.js can import from sharedFunctions.js without depending on each other.
2. Use Dependency Injection
Instead of importing modules directly, you can pass them as arguments.
// fileA.js
export function functionA(dependency) {
dependency();
}
This makes the module more flexible and avoids tight coupling between files.
3. Break Large Files into Smaller Ones
If a file does too many things, it’s more likely to cause dependency problems. Break it into smaller files that handle one task each.
4. Use Index Files Carefully
An index.js file in a folder can be helpful to group exports. But if it imports everything inside the folder and other files in the folder import from it, you can easily create a loop.
Avoid circular imports in index.js by keeping exports simple and avoiding deep links back into the folder.
5. Use Interfaces or Types Separately
In TypeScript, you can define interfaces and types in their own file. This allows both frontend and backend to use the same types without importing logic-heavy files.
This is especially useful in full stack projects that share types between server and client.
Best Practices to Prevent Circular Dependencies
Here are some practical tips to help you avoid circular dependencies from the start:
- Keep your files small and focused.
- Use clear folder structures: models, services, routes, utils.
- Avoid importing files unless necessary.
- Create a shared/ folder for reusable logic or types.
- Use tools like madge in CI pipelines to catch problems early.
- Write unit tests to catch unexpected behavior from broken imports.
Organizing Full Stack Code to Avoid Circular Dependencies
In full stack projects, both the frontend and backend may need access to shared logic. Here’s one way to structure your project safely:
/project
/client
/server
/shared
/utils
/types
- client/ contains only browser code
- server/ contains only backend code
- shared/ contains safe modules used by both
Each part of the app only imports what it needs, and shared logic is kept separate and clean.
If you’re learning through full stack developer classes, try practicing this structure in your personal projects. It helps you write clean, scalable code from the beginning.
Conclusion
Circular dependencies can cause serious problems in full stack applications, especially when the project becomes large and complex. They can lead to broken imports, undefined values, slow performance, and confusing bugs.
But with the right tools, smart file structure, and good habits, you can detect, fix, and prevent circular dependencies. Use tools like madge, keep your files small and modular, and think about how your modules depend on each other.
If you’re currently taking a full stack course, now is an excellent time to start using these practices. Understanding circular dependencies and how to manage them will not only help you avoid bugs but will also make you a more confident and capable developer.
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