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.