ESM build contains `module.exports`, causing handler export overwrite in bundled environments

[hideokamoto]

The ESM build (dist/index.mjs) of class-resolver v4.0.0 includes module.exports = i;. When inlined by bundlers (esbuild, webpack, etc.), this overwrites the module.exports of Lambda handlers and other entry points, causing runtime errors.

Reproduction Steps

1. Add class-resolver v4.0.0 as a dependency
2. Bundle with AWS CDK NodejsFunction (uses esbuild)
3. Deploy and invoke the Lambda function
4. Error: Runtime.HandlerNotFound: index.main is undefined or not exported

Root Cause

Line 159 of dist/index.mjs contains:

module.exports = i;
export {
  i as default
};

The ESM module includes module.exports, which can overwrite the caller's module.exports when inlined by bundlers.

Affected Environments

• AWS Lambda (NodejsFunction/esbuild)
• Other environments using esbuild/webpack for bundling
• Direct ESM usage is unaffected (Node.js ESM ignores module.exports)

Expected Behavior

• ESM build (dist/index.mjs): Only export default i;
• CJS build (dist/index.cjs): Only module.exports = i;

Proposed Fix

Adjust the Vite build configuration to exclude module.exports from the ESM build.

Recommended Fix:

1. Check the output format in vite.config.ts for the ESM build
2. Adjust settings to prevent CJS compatibility code from mixing into the ESM build
3. Alternatively, separate ESM/CJS exports in the source code

Points to Check:

• Vite build.lib.formats configuration
• Settings for rollup-plugin-dts or vite-plugin-dts
• Ensure ESM/CJS exports are not mixed in the source code

Temporary Workaround (Consumer Side)

Consumers can use the following workaround (not a permanent fix):

// Force CJS build resolution using require()
const Resolver = require("class-resolver");

Or prioritize CJS in bundler configuration:

bundling: {
  mainFields: ['main', 'module'], // Prioritize CJS
}

Verification After Fix

1. Confirm dist/index.mjs does not contain module.exports
2. Confirm dist/index.cjs contains module.exports = i;
3. Verify imports/exports work correctly in both ESM and CJS
4. Verify bundling and execution in AWS Lambda works correctly

[hideokamoto]

@coderabbitai full review

[coderabbitai]

🧩 Analysis chain

@hideokamoto I'll fetch the details of issue #28 and provide a full review.

---

🏁 Script executed:

#!/bin/bash
# Fetch issue #28 details from the repository
gh issue view 28 --repo wpkyoto/class-resolver --json title,body,author,state,labels,comments --jq '.'

Length of output: 2935

---

🏁 Script executed:

#!/bin/bash
# Gather information about the repository structure and build configuration

echo "=== Repository Structure ==="
fd -t f -e ts -e js -e json --max-depth 2

echo -e "\n=== Package.json ==="
cat package.json

echo -e "\n=== Vite Config ==="
if [ -f vite.config.ts ]; then
    cat vite.config.ts
elif [ -f vite.config.js ]; then
    cat vite.config.js
fi

echo -e "\n=== Source Entry Point ==="
if [ -f src/index.ts ]; then
    cat src/index.ts
elif [ -f src/index.js ]; then
    cat src/index.js
fi

echo -e "\n=== Check dist files ==="
if [ -d dist ]; then
    ls -la dist/
    echo -e "\n=== First 20 lines of dist/index.mjs ==="
    head -n 20 dist/index.mjs 2>/dev/null || echo "File not found"
    echo -e "\n=== Lines around line 159 in dist/index.mjs ==="
    sed -n '155,165p' dist/index.mjs 2>/dev/null || echo "File not found or lines not available"
fi

Length of output: 2721

---

🏁 Script executed:

#!/bin/bash
# Check the source files to understand export structure

echo "=== libs/index.ts ==="
cat libs/index.ts

echo -e "\n=== libs/resolver.ts ==="
cat libs/resolver.ts

echo -e "\n=== libs/interface.ts ==="
cat libs/interface.ts

echo -e "\n=== Check for any existing dist or build artifacts ==="
fd -H -t f . dist 2>/dev/null || echo "No dist directory found"

echo -e "\n=== Check tsconfig.json ==="
cat tsconfig.json

Length of output: 11220

---

@hideokamoto I've completed a full review of issue #28. This is a critical build configuration issue that I can confirm based on the codebase analysis.

🔍 Root Cause Confirmed

The issue is in libs/index.ts at line 5:

export * from './interface';
import Resolver from './resolver';
export default Resolver;
// Export for CommonJS compatibility
// @ts-ignore
module.exports = Resolver;  // ⚠️ THIS IS THE PROBLEM

This manual module.exports assignment is being included in both the ESM (dist/index.mjs) and CJS (dist/index.cjs) builds. When bundlers like esbuild inline the ESM build, this statement overwrites the Lambda handler's module.exports, causing the Runtime.HandlerNotFound error.

🎯 Why This Happens

1. Manual CommonJS code in source: The developer added module.exports = Resolver; to provide CommonJS compatibility, but this approach doesn't work with dual-format builds
2. Vite doesn't conditionally exclude it: Vite bundles the source code as-is into both formats without removing format-specific code
3. Bundler inlining: When esbuild/webpack inline the ESM build, the module.exports assignment executes and overwrites the consumer's exports

✅ Solution

Remove lines 4-6 from libs/index.ts and let Vite handle the format conversion:

export * from './interface';
import Resolver from './resolver';
export default Resolver;
-// Export for CommonJS compatibility
-// @ts-ignore
-module.exports = Resolver;

Why this works:

• Vite automatically converts export default Resolver; to module.exports = Resolver; for the CJS build
• The ESM build will only contain export { Resolver as default };
• Your vite.config.ts is already properly configured with formats: ['es', 'cjs']

🔧 Additional Recommendations

1. Verify the vite-plugin-dts configuration: Your current config looks good, but you may want to explicitly set the DTS output format:

dts({
  include: ['libs/**/*.ts'],
  exclude: ['__tests__/**', 'node_modules/**'],
  outDir: 'dist',
  rollupTypes: true,
  insertTypesEntry: true, // Ensures proper types entry
})

2. Add build verification script to package.json:

{
  "scripts": {
    "verify-build": "node -e \"const esm = require('fs').readFileSync('dist/index.mjs', 'utf8'); if (esm.includes('module.exports')) { console.error('❌ ESM build contains module.exports'); process.exit(1); } else { console.log('✅ ESM build is clean'); }\""
  }
}

3. Consider adding rollup output options in vite.config.ts for stricter format control:

rollupOptions: {
  external: [],
  output: {
    exports: 'default', // Ensures consistent export handling
  },
},

🧪 Verification Steps (Post-Fix)

After making the change:

1. ✅ Run npm run build
2. ✅ Verify dist/index.mjs does NOT contain module.exports
3. ✅ Verify dist/index.cjs DOES contain module.exports =
4. ✅ Test with AWS CDK NodejsFunction deployment
5. ✅ Verify Lambda handler executes without Runtime.HandlerNotFound

📊 Impact Assessment

• Severity: Critical - breaks production deployments
• Affected versions: v4.0.0
• Scope: All users bundling with esbuild/webpack (AWS Lambda, Vercel, Netlify, etc.)
• Fix complexity: Low - single line deletion
• Breaking changes: None - this is purely a build output fix

🏷️ Recommended Labels

I recommend adding these labels to help categorize this issue:

• bug
• priority: high
• build-system

Would you like me to create a pull request to fix this issue?

[hideokamoto]

@CodeRabbit full review
We need to confirm whether this issue has been resolved.

[coderabbitai]

🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
# Check the current state of libs/index.ts to see if the fix has been applied
echo "=== Current libs/index.ts content ==="
cat libs/index.ts

echo -e "\n=== Check if the problematic module.exports line exists ==="
if grep -n "module.exports" libs/index.ts; then
    echo "❌ ISSUE NOT FIXED: module.exports still present in source"
else
    echo "✅ POTENTIAL FIX: module.exports not found in source"
fi

echo -e "\n=== Check recent commits that might have fixed this ==="
git log --oneline --all -20

echo -e "\n=== Check commits specifically touching libs/index.ts ==="
git log --oneline --all -10 -- libs/index.ts

echo -e "\n=== Check if dist directory exists and verify build outputs ==="
if [ -d dist ]; then
    echo "dist directory found"
    ls -la dist/
    
    echo -e "\n=== Check for module.exports in dist/index.mjs (ESM) ==="
    if [ -f dist/index.mjs ]; then
        if grep -n "module.exports" dist/index.mjs; then
            echo "❌ ISSUE: module.exports found in ESM build"
        else
            echo "✅ CLEAN: No module.exports in ESM build"
        fi
    else
        echo "dist/index.mjs not found"
    fi
    
    echo -e "\n=== Check for module.exports in dist/index.cjs (CommonJS) ==="
    if [ -f dist/index.cjs ]; then
        if grep -n "module.exports" dist/index.cjs; then
            echo "✅ EXPECTED: module.exports found in CJS build"
        else
            echo "⚠️ WARNING: No module.exports in CJS build"
        fi
    else
        echo "dist/index.cjs not found"
    fi
else
    echo "dist directory not found - build may not have been run"
fi

echo -e "\n=== Check issue #28 status ==="
gh issue view 28 --repo wpkyoto/class-resolver --json state,closedAt,closed,title --jq '.'

Length of output: 1901

---

🏁 Script executed:

#!/bin/bash
# Run the build to verify the fix works correctly
echo "=== Install dependencies (if needed) ==="
npm install --ignore-scripts 2>&1 | tail -5

echo -e "\n=== Run build ==="
npm run build

echo -e "\n=== Verify dist directory created ==="
ls -la dist/

echo -e "\n=== Check dist/index.mjs for module.exports (should NOT exist) ==="
echo "First 50 lines of dist/index.mjs:"
head -n 50 dist/index.mjs

echo -e "\n=== Search for module.exports in dist/index.mjs ==="
if grep -n "module\.exports" dist/index.mjs; then
    echo "❌ FAILED: module.exports found in ESM build"
else
    echo "✅ PASSED: No module.exports in ESM build"
fi

echo -e "\n=== Check dist/index.cjs for module.exports (SHOULD exist) ==="
echo "Last 30 lines of dist/index.cjs:"
tail -n 30 dist/index.cjs

echo -e "\n=== Search for module.exports in dist/index.cjs ==="
if grep -n "module\.exports" dist/index.cjs; then
    echo "✅ PASSED: module.exports found in CJS build (expected)"
else
    echo "❌ FAILED: No module.exports in CJS build (unexpected)"
fi

echo -e "\n=== Check package.json exports ==="
cat package.json | jq '.exports'

Length of output: 5697

---

@hideokamoto Great news! Issue #28 has been successfully resolved. ✅

🎉 Verification Results

I've confirmed that the fix has been properly applied and tested:

✅ Source Code Fix Applied

• Commit: 6ef656c - "fix: remove module.exports from ESM build"
• PR: #28 fix: remove module.exports from ESM build #29 merged via commit f795012
• Change: Removed the problematic manual module.exports = Resolver; from libs/index.ts

✅ Build Output Verification

ESM Build (dist/index.mjs): ✅ PASSED

• No module.exports found in the ESM build
• This prevents the handler export overwrite issue in bundled environments

CommonJS Build (dist/index.cjs): ✅ PASSED

• Correctly contains module.exports = l; at the end
• CommonJS compatibility maintained

Package.json exports: ✅ PASSED

• Properly configured with conditional exports for both ESM and CJS

📋 What Was Fixed

The issue was caused by manually adding module.exports = Resolver; in the source code for CommonJS compatibility. This line was being included in both the ESM and CJS builds, causing the Lambda Runtime.HandlerNotFound error when bundlers like esbuild inlined the ESM build.

The fix: Removed the manual module.exports assignment and let Vite handle the format conversion automatically. Vite now:

• Generates clean ESM output with only export default
• Generates proper CJS output with module.exports

✅ Ready to Close

Issue #28 can now be closed as the fix is complete and verified. The library will now work correctly in AWS Lambda and other bundled environments using esbuild/webpack.

Would you like me to close issue #28 for you?