Build tool supports tree shaking (no barrel file issues)

Barrel files that re-export every module in a directory prevent the build tool from eliminating dead code — even when you only import one function, the bundler may include the entire barrel. A lib/index.ts that re-exports 50 utilities causes every importer to pull in all 50, regardless of usage. Without sideEffects: false in package.json, the bundler cannot safely drop any of those exports. On large codebases, this pattern adds tens to hundreds of kilobytes to the bundle that serve no user-visible function. ISO 25010 performance-efficiency.resource-utilization identifies this as avoidable resource inclusion.

Set sideEffects: false in package.json to signal the bundler that modules can be safely dropped when unused. Reduce or eliminate barrel files with more than 10 re-exports, replacing them with direct imports.

// package.json
{
  "sideEffects": false
}

// Before — barrel file prevents tree shaking
// lib/index.ts
export * from './utils/date'
export * from './utils/string'
export * from './hooks/useCounter'
export * from './hooks/useDebounce'
export * from './components/Button'
// ...40 more

// After — import directly from the module
import { formatDate } from '@/lib/utils/date'
import { useCounter } from '@/lib/hooks/useCounter'

For monorepos, also set sideEffects: false in each package's package.json, not just the root.

• ID: performance-load.bundle.tree-shaking

• Severity: low

• What to look for: Count all barrel files (index.ts/index.js that re-export from other modules) in the project. For each barrel file, count the number of re-exports. Check for sideEffects: false in package.json, or examine build config for tree-shaking enablement. Modern frameworks like Next.js enable tree-shaking by default. Enumerate: "X barrel files found with Y total re-exports."

• Pass criteria: The build tool supports tree shaking (verified by sideEffects: false in package.json or framework default), and no barrel file re-exports more than 20 items without the module being fully consumed. No more than 3 barrel files exist that re-export 10+ items. Report: "X barrel files found, largest re-exports Y items. Tree shaking enabled via [mechanism]."

• Fail criteria: Tree shaking is disabled (explicit sideEffects: true in package.json or build tool configured without tree shaking), or more than 3 barrel files each re-export 10+ items without proper configuration.

• Skip (N/A) when: Never — tree shaking reduces bundle size.

• Detail on fail: "package.json has sideEffects: true — prevents tree shaking across 5 barrel files with 80+ total re-exports" or "4 barrel files in lib/ re-export 15+ items each — prevents unused export elimination"

• Remediation: Enable tree shaking:

  {
    "sideEffects": false
  }
  

  Minimize barrel files:

  // Before — barrel file prevents tree shaking
  // lib/index.ts
  export * from './utils'
  export * from './hooks'
  export * from './components'
  
  // After — granular imports
  // Only import what you need
  import { useCounter } from './hooks/useCounter'
  import { formatDate } from './utils/date'