# Unused assets and dead code excluded from production build

- **Pattern:** `ab-001374` (`extension-ux-performance.bundle-memory.no-unused-assets`)
- **Severity:** low
- **Lifecycle:** active
- **Last modified:** 2026-04-18
- **Canonical URL:** https://auditbuffet.com/patterns/ab-001374
- **License:** CC-BY-4.0 — attribute to AuditBuffet Pattern Catalog (https://auditbuffet.com/patterns/ab-001374)

## Why it matters

Unused assets and dead code bundled into a production extension inflate the download size every user pays on install and update, slow popup initialization as JavaScript is parsed, and expose development-only test fixtures or debug data that could leak internal implementation details. ISO 25010:2011 performance-efficiency.resource-utilisation applies: every unreferenced file is pure overhead. Source maps included in packaged extensions additionally expose original source code to any user who inspects the `.crx`.

## Severity rationale

Low because unused assets inflate bundle size and can expose internal paths, but do not directly break functionality or create immediate security risk.

## Remediation

Configure your bundler to exclude source maps, screenshots, and test fixtures from the production copy, and verify all remaining assets are referenced in `manifest.json` or code.

```js
// webpack.config.js — exclude development artifacts from production
const CopyPlugin = require('copy-webpack-plugin');

module.exports = {
  devtool: false,
  plugins: [
    new CopyPlugin({
      patterns: [{
        from: 'public',
        globOptions: {
          ignore: ['**/*.map', '**/screenshots/**', '**/test-fixtures/**'],
        },
      }],
    }),
  ],
};
```

After building, run `find dist/ -type f | sort -k1 -t/ | xargs ls -lh` and cross-reference every file against `manifest.json` icon declarations and HTML `<script>`/`<link>` tags. Remove anything with no reference.

## Detection

- **ID:** `no-unused-assets`
- **Severity:** `low`
- **What to look for:** Examine the build output directory for assets that are not referenced by any manifest field, HTML file, or JavaScript file. Look for: duplicate icon sizes not declared in manifest `icons`, old screenshots or demo images accidentally committed to the extension root, development-only test fixtures included in the build, commented-out feature folders with assets, and build artifacts like `.map` files in the production package. Also check for dead JavaScript code — exported functions or modules that are never imported. List all assets in the extension directory and enumerate which are referenced in code vs. which are orphaned. Report: X of Y assets are referenced.
- **Pass criteria:** All files in the production build directory are referenced by the extension. No `.map` source map files included in the packaged extension. No demo data, test fixtures, or unused icon sizes present. Tree-shaking is enabled in the bundler config. At least 1 implementation must be confirmed.
- **Fail criteria:** Unreferenced assets (images, fonts, JSON files) are present in the build output. Source map files included in the packaged extension. Development fixtures or sample data bundled with the production build.
- **Skip (N/A) when:** Build output directory is not present in the codebase (source-only repository).
- **Detail on fail:** List the unreferenced assets. Example: `"dist/ contains screenshots/demo.png (1.2MB) not referenced by manifest or any HTML file"` or `"Production build includes 6 .map files totaling 3.4MB — source maps should be excluded from packaged extension."`
- **Remediation:** Configure your bundler to exclude development artifacts:

  ```js
  // webpack.config.js
  const CopyPlugin = require('copy-webpack-plugin');

  module.exports = {
    plugins: [
      new CopyPlugin({
        patterns: [
          {
            from: 'public',
            globOptions: {
              ignore: ['**/*.map', '**/screenshots/**', '**/test-fixtures/**'],
            },
          },
        ],
      }),
    ],
    devtool: false,  // No source maps in production
  };
  ```

  Audit your `manifest.json` icon entries and remove icon sizes not declared there from the production copy.

---

## External references

- iso-25010 performance-efficiency.resource-utilization

Taxons: performance

HTML version: https://auditbuffet.com/patterns/ab-001374