Introduction
Recently, while managing files in a Monorepo, I discovered an interesting export management pattern: Barrel File. Therefore, I researched this method’s philosophy and pros and cons to structure better projects.
What is Barrel File Pattern?
A Barrel File in TypeScript or JavaScript refers to grouping multiple modules into a single file, making it easier to have a unified interface for imports.
For example, if there are three modules (moduleA.js
, moduleB.js
, moduleC.js
), to manage them uniformly using a Barrel File, you can create an additional index.js
that imports these three modules, and then use index.js
as the interface for these three modules.
export { default as moduleA } from './moduleA.js';export { default as moduleB } from './moduleB.js';export { default as moduleC } from './moduleC.js';
You can then directly import this Barrel File to obtain the above three modules at once:
import { value1, value2, value3 } from './foobar';
The benefits of this pattern are clear; it effectively manages module exports, enhances the readability of module imports, and creates a clean and consistent interface for external use, especially common in Icon libraries or component libraries.
Potential Issues
- Development Experience Concerns: Barrel Files may prevent developers from clearly seeing the source of modules, especially in large projects with multiple layers of Barrel Files. Not all development environments support easy navigation to the source of imported files, and multi-layered output imports can cause confusion.
- Build and Analysis Overhead: If the bundler used does not support or is not correctly configured for Tree Shake (removing unused code), all modules that are not actually used will still be bundled, increasing the amount of unused code. Even with a supported bundler, it will need to analyze the structure of the Barrel File to correctly remove unused modules.
- Testing and Linter Overhead: Testing may require loading each module every time a module is accessed through a Barrel File, increasing test time. Import detection-related Linters may also increase the complexity of checks due to Barrel Files.
Conclusion
Some people believe Barrel Files are an Anti-Pattern with no benefits, while others think this approach can better organize modules, unify interfaces, and enhance readability.
My view is that if your Bundler is powerful and supports Tree Shake, and team members agree with this pattern, then Barrel Files are a viable option. For example, Vercel wrote an article describing how Next.js overcomes issues caused by Barrel Files: How we optimized package imports in Next.js - Vercel
The reason for exploring this topic is that my current team heavily uses this pattern, but Vite does not perform Tree Shake during development, leading to a lot of unused code being bundled, severely affecting the development experience.
Further Reading
- Barrel files in JavaScript - flaming.codes
- Why you should avoid Barrel Files in JavaScript Modules? - Bart
- Please Stop Using Barrel Files - TkDodo’s blog