In this article, we will analyze why Rollup encounters circular dependency errors, explore debugging techniques, and provide best practices to resolve and prevent circular imports in modern JavaScript applications.

Understanding Circular Dependencies in Rollup

Circular dependencies occur when two or more modules import each other directly or indirectly. While ES modules allow circular imports, Rollup’s static analysis may struggle to resolve them correctly, leading to unexpected behavior.

Common Causes

• Two modules importing each other explicitly.
• Indirect circular imports through multiple dependency chains.
• Improper module re-exports causing recursive references.
• Use of export * leading to unintended import loops.

Common Symptoms

• Rollup build fails with "Circular dependency detected" warnings.
• Application crashes due to undefined module exports.
• Unexpected behavior, such as functions being undefined at runtime.
• Duplicate modules in the output bundle.

Diagnosing Circular Dependency Issues

1. Enabling Debug Mode

Enable verbose logging in Rollup to inspect dependency resolution:

rollup --config --logLevel debug

2. Checking Circular Dependency Warnings

Run Rollup with the --silent flag to isolate warnings:

rollup --config --silent

Look for warnings similar to:

(!) Circular dependency: src/moduleA.js -> src/moduleB.js -> src/moduleA.js

3. Visualizing Dependency Graphs

Use the rollup-plugin-visualizer to inspect module imports:

import visualizer from 'rollup-plugin-visualizer';

export default {
  plugins: [
    visualizer({ filename: 'bundle-stats.html' })
  ]
};

Run the build and analyze bundle-stats.html for circular imports.

Fixing Circular Dependencies in Rollup

Solution 1: Refactoring Direct Circular Imports

If two modules import each other, refactor shared logic into a third module.

Before (Circular Dependency):

// moduleA.js
import { functionB } from './moduleB.js';
export function functionA() {
  functionB();
}

// moduleB.js
import { functionA } from './moduleA.js';
export function functionB() {
  functionA();
}

After (Refactored Solution):

// sharedModule.js
export function sharedLogic() {
  console.log('Shared logic');
}

// moduleA.js
import { sharedLogic } from './sharedModule.js';
export function functionA() {
  sharedLogic();
}

// moduleB.js
import { sharedLogic } from './sharedModule.js';
export function functionB() {
  sharedLogic();
}

Solution 2: Using Dynamic Imports

For cases where circular imports cannot be avoided, use dynamic import() to break the dependency loop.

export async function functionA() {
  const { functionB } = await import('./moduleB.js');
  functionB();
}

Solution 3: Avoiding export * Statements

Wildcard exports can introduce unintended circular references. Replace:

export * from './moduleA.js';
export * from './moduleB.js';

With explicit named exports:

export { functionA } from './moduleA.js';
export { functionB } from './moduleB.js';

Solution 4: Using rollup-plugin-commonjs for Compatibility

If using CommonJS dependencies, add rollup-plugin-commonjs to ensure proper resolution:

import commonjs from '@rollup/plugin-commonjs';
export default {
  plugins: [commonjs()]
};

Best Practices for Avoiding Circular Dependencies

• Refactor shared logic into standalone modules.
• Use dynamic imports (import()) for lazy loading dependencies.
• Avoid wildcard exports that introduce unnecessary dependencies.
• Use dependency visualization tools to track import cycles.
• Break dependency chains by restructuring module relationships.

Conclusion

Circular dependencies in Rollup can lead to failed builds and runtime errors. By restructuring module imports, using dynamic imports, and avoiding problematic re-exports, developers can ensure a smooth and optimized Rollup build process.

FAQ

1. How do I detect circular dependencies in Rollup?

Use rollup --config --silent to check for circular dependency warnings.

2. Why does Rollup fail to bundle when circular imports exist?

Rollup’s static analysis may not resolve circular imports properly, leading to undefined exports.

3. Can circular dependencies be fixed without refactoring?

In some cases, using import() or rollup-plugin-commonjs can break dependency loops.

4. How do I visualize module dependencies in Rollup?

Use rollup-plugin-visualizer to generate an interactive dependency graph.

5. What’s the best way to prevent circular dependencies?

Modularize shared logic, avoid wildcard exports, and use dynamic imports when necessary.