Barrel Exports Analyzer & Migrator

Detects barrel export anti-patterns and safely migrates to direct imports. Every recommendation is backed by real experiment data: barrel imports produced a 224 KB page chunk vs 11.7 KB with direct imports — a 19x difference in a controlled Next.js monorepo test. When presenting results, always include both your own measurements from the user's project and this 19x reference data point for context.

How It Works

This skill operates in two phases:

1. Analysis — Find barrel files, detect side effects, check bundler config, measure bundle impact
2. Migration — Rewrite imports file-by-file with user confirmation, update config, verify with before/after metrics

You can run analysis alone (read-only, always safe) or follow up with migration.

Phase 1: Analysis

When the user asks to analyze barrels, check barrel exports, or investigate bundle size, follow these four steps.

Step 1: Find Barrel Files

Search for index.ts and index.tsx files that primarily re-export from other modules:

# Find all index files
find src/ -name "index.ts" -o -name "index.tsx" | head -50

For each file found, read it and classify using exactly these terms in your report:

• Pure barrel — contains ONLY export * from or export { X } from statements
• Mixed barrel — has re-exports AND original code (types, constants, logic)
• Entry point — has original code, not primarily re-exports
• Side-effect barrel — has re-exports AND side effects (console.log, global mutations, event listeners)

Always use this taxonomy when reporting — it helps users understand the severity and gives them a shared vocabulary for discussing barrel patterns.

For the complete classification algorithm and side-effect taxonomy, see references/analysis-guide.md

Step 2: Check Configuration

Inspect the project's bundler and package configuration. Go through every item in this checklist — don't skip any:

• package.json "exports" — subpath exports enable direct imports. If missing, this is a key recommendation.
• package.json "sideEffects" — tells the bundler what can be safely tree-shaken. If missing and barrels have no side effects, recommend adding "sideEffects": false.
• package.json "type": "module" — ESM enables better tree-shaking
• next.config.js/ts optimizePackageImports (if Next.js) — Next.js's built-in barrel optimization. Always check for this explicitly and report whether it's present or missing.
• next.config.js/ts transpilePackages — required for monorepo internal packages
• tsconfig.json "moduleResolution" — must be "bundler" or "node16" for subpath exports

Step 3: Measure Bundle (Next.js)

If the project uses Next.js, measure the current bundle:

npx next build 2>&1

Parse the route table from build output. Look for the First Load JS column — this is the total JS sent to the browser for each route. Record the baseline values.

Key metrics to capture:

• First Load JS for each route
• Shared chunk size (listed under + First Load JS shared by all)
• Individual page chunk sizes

Step 4: Report Findings

Present findings in this format:

## Barrel Export Analysis

### Barrel Files Found: N

| File | Type | Exports | Side Effects | Risk |
|------|------|---------|-------------|------|
| src/index.ts | Pure barrel | 45 | None | High |
| src/components/index.ts | Pure barrel | 20 | None | Medium |
| src/utils/index.ts | Mixed | 12 | console.log | Critical |

### Configuration Issues

- ⚠️ No subpath exports configured
- ✅ `transpilePackages` includes internal packages

### Bundle Impact (if measured)

- Current First Load JS: 155 kB
- Estimated savings: 30-50% (based on barrel depth and export count)

### Recommendations

1. [Specific actions based on findings]

Risk levels:

• Critical — barrel with side effects, bundler cannot tree-shake
• High — barrel re-exporting 20+ modules, high import fan-out
• Medium — barrel with 5-20 re-exports
• Low — barrel with <5 re-exports or already optimized via config

Phase 2: Migration

When the user asks to migrate barrels, fix barrel exports, or optimize imports, follow these six steps. Always run Phase 1 first if analysis hasn't been done.

Step 1: Establish Baseline

Always run a build and record baseline metrics BEFORE touching any imports. Without a baseline, you cannot prove the impact of the migration. This step is not optional.

npx next build 2>&1

Save the First Load JS values and page chunk sizes. You will compare against these numbers after migration to show the user concrete before/after improvement.

Step 2: Find Consumers

For each barrel file identified in analysis, find all files that import from it:

# Find all imports from a barrel path
grep -rn "from ['\"]@repo/ui['\"]" src/ --include="*.ts" --include="*.tsx"
grep -rn "from ['\"]./components['\"]" src/ --include="*.ts" --include="*.tsx"

Build a mapping: { consumer_file → [imported_names] → barrel_path → direct_path }.

Step 3: Verify Direct Paths Exist

Before rewriting any imports, verify that each target module file exists:

@repo/ui → @repo/ui/components/Button  →  packages/ui/src/components/Button.tsx ✓
@repo/ui → @repo/ui/hooks/useDebounce  →  packages/ui/src/hooks/useDebounce.ts  ✓

If subpath exports are configured in the package's package.json, use those paths. Otherwise, use relative paths.

Step 4: Apply Changes (With Confirmation)

CRITICAL: Never auto-apply all changes. Process file by file.

For each consumer file, show the user the proposed change:

File: src/app/page.tsx

BEFORE:
import { Button, Card, Badge, IconHome, useDebounce, formatDate, cn } from "@repo/ui";

AFTER:
import { Button } from "@repo/ui/components/Button";
import { Card } from "@repo/ui/components/Card";
import { Badge } from "@repo/ui/components/Badge";
import { IconHome } from "@repo/ui/icons/IconHome";
import { useDebounce } from "@repo/ui/hooks/useDebounce";
import { formatDate } from "@repo/ui/utils/formatDate";
import { cn } from "@repo/ui/utils/cn";

Wait for user confirmation before applying each file. Group related files if the user prefers batch mode.

For detailed transformation rules covering type imports, namespace imports, re-exports, and edge cases, see references/migration-guide.md

Step 5: Update Configuration

After import migration, review and update configuration. Even if some fields already exist, verify they cover all the direct paths you need:

package.json exports (of the library being imported):

• If the field doesn't exist, add it:

{
  "exports": {
    ".": "./src/index.ts",
    "./components/*": "./src/components/*.tsx",
    "./hooks/*": "./src/hooks/*.ts",
    "./utils/*": "./src/utils/*.ts"
  }
}

• If it already exists, verify it covers all the subpaths your migration uses. Show the user the current exports and confirm they match the direct import paths. If wildcards are used (./components/*), verify the source files follow the expected naming convention.

next.config.js (if Next.js, for external packages):

experimental: {
  optimizePackageImports: ["@repo/ui"]
}

Step 6: Post-Migration Verification

Run the build again and compare:

## Migration Results

| Metric | Before | After | Change |
|--------|--------|-------|--------|
| First Load JS (/) | 155 kB | 106 kB | -31.6% |
| Page chunk (/) | 224 kB | 11.7 kB | -94.8% |

Verify:

• All routes build without errors
• No runtime errors (if dev server available)
• TypeScript compilation passes: npx tsc --noEmit
• Bundle size decreased or stayed the same

Edge Cases

Guidelines

• Never auto-apply changes — always show the diff and get confirmation
• Preserve type imports — import type has zero runtime cost, migration is optional
• Handle external packages differently — use optimizePackageImports for third-party, rewrite for internal
• Always measure — run builds before AND after to prove impact with numbers
• One file at a time — process consumer files individually unless user requests batch mode
• Keep barrel files initially — don't delete index.ts until all consumers are migrated
• Check for side effects first — if a barrel has side effects, migration requires more care

Usage Examples

"My Next.js app bundle is too big"

1. Run analysis to find barrel files and measure current bundle
2. Identify the highest-impact barrels (most re-exports, most consumers)
3. Show expected savings based on experiment data
4. Offer to migrate the top offenders first

"Check if my project has barrel export problems"

1. Run Phase 1 analysis only
2. Report all barrel files with risk levels
3. Check configuration for missing optimizations
4. Provide specific recommendations without making changes

"Migrate all my barrel imports to direct imports"

1. Run Phase 1 if not already done
2. Establish build baseline
3. Process each consumer file with confirmation
4. Update package.json exports
5. Run post-migration build and show before/after comparison

Reference Materials

• references/analysis-guide.md — Detection methodology, side-effect taxonomy, and reporting format
• references/migration-guide.md — Import transformation rules, edge cases, and rollback procedures
• references/barrel-patterns.md — 7 anti-patterns catalog with experiment data and decision matrix