What Are Barrel Files, and Why Should You Avoid Them?

If you’ve worked on JavaScript/TypeScript projects, chances are you’ve already encountered barrel files, even if you didn’t know their name.

A barrel file is essentially an index.js or index.ts file used to re-export modules from a directory, like this:

// components/index.ts
export { Button } from './Button';
export { Card } from './Card';
export { Modal } from './Modal';        

This allows for simplified imports:

import { Button, Card, Modal } from './components';        

At first glance, barrel files seem like an excellent tool to improve developer experience. But my recent experience has shown how they can become a hidden performance bottleneck, especially in front-end projects.

My Experience

I’m currently working on a React.js web app using Vite as the bundler. Recently, Vite started warning me that the initial bundle size for our app was too large—an alarming 4MB.

To identify the cause, I conducted a proof of concept:

1. Step 1: I removed all dependencies from the entry point file, leaving only a single internal dependency. After building the project again, the initial bundle size dropped slightly but was still 3MB.
2. Step 2: I updated all imports in that dependency to use direct imports instead of the barrel file.
3. The Result: The initial bundle size dropped to just 327KB.

This experiment made it clear: barrel files were the root of the problem.

Why Barrel Files Can Be Dangerous

Barrel files can cause bundlers to inadvertently include unnecessary modules, even if your code doesn’t use them. This bloats the bundle size and negatively impacts performance, especially for front-end apps where load times are critical.

Best Practices to Avoid Barrel File Pitfalls

• Use Direct Imports: Always import what you need directly:

import { Button } from './components/Button';        

• Ensure Proper Tree Shaking: Make sure your bundler is configured to eliminate unused code.
• Use Barrels Sparingly: If you do use barrel files, limit them to non-critical or server-side modules.

Key Takeaway

Barrel files can simplify imports, but they come with a cost. My experience demonstrates how they can stealthily contribute to oversized bundles and performance issues.

?? Have you encountered similar challenges with barrel files or other bundling issues? Let’s discuss in the comments!