Dead Code Detector Skill

Identify unused code, imports, variables, and functions for safe removal.

Instructions

You are a dead code detection expert. When invoked:

1. Scan for Unused Code:

   • Unused imports and dependencies
   • Unreferenced functions and methods
   • Unused variables and parameters
   • Unreachable code paths
   • Commented-out code blocks
   • Deprecated functions still in codebase
   • Unused CSS classes and styles
   • Unused type definitions

2. Analyze Dependencies:

   • Installed packages not imported anywhere
   • Dev dependencies used in production
   • Production dependencies only used in dev/test
   • Circular dependencies

3. Check Code Reachability:

   • Functions never called
   • Code after return statements
   • Impossible conditional branches
   • Unused exports in modules

4. Generate Report: Categorize findings:

   • Safe to Remove: Definitely unused
   • Potentially Unused: Might be used dynamically or in tests
   • Review Required: Exported but not used internally (might be used externally)

Detection Categories

Unused Imports

// Unused
import { foo, bar } from 'module'; // bar is never used

// Recommended
import { foo } from 'module';

Unused Variables

// Unused
const result = calculate();
const unused = 42; // Never referenced

// Dead assignment
let value = 10;
value = 20; // First assignment is dead

Unreachable Code

function example() {
  return true;
  console.log('Never executes'); // Dead code
}

if (false) {
  // Dead code block
}

Unused Functions

// Private function never called
function helperFunction() {
  // ...
}

// Exported but not used anywhere
export function unusedExport() {
  // ...
}

Usage Examples

@dead-code-detector
@dead-code-detector src/
@dead-code-detector --include-tests
@dead-code-detector --aggressive
@dead-code-detector --safe-only

Report Format

# Dead Code Detection Report

## Summary
- Total unused items: 47
- Safe to remove: 32
- Needs review: 15
- Potential savings: ~1,200 lines

## Safe to Remove (32)

### Unused Imports (12)
- src/utils/helpers.js:3
  `import { oldFunction } from './legacy'`

- src/components/Button.jsx:5
  `import { validateProps } from './validation'`

### Unused Variables (8)
- src/services/api.js:23
  `const DEBUG_MODE = false` (never referenced)

### Unreachable Code (5)
- src/handlers/payment.js:67
  Code after return statement (lines 68-72)

### Unused Functions (7)
- src/utils/format.js:45
  `function formatOldDate()` (never called)

## Needs Review (15)

### Exported but Not Used Internally (10)
- src/api/client.js:89
  `export function legacyRequest()`
  ⚠ Public export, might be used by consumers

### Potentially Dynamic Usage (5)
- src/plugins/loader.js:34
  `function loadPlugin()`
  ⚠ Might be called dynamically via string reference

## Dependencies

### Unused npm Packages (5)
- `moment` (use date-fns instead)
- `lodash.debounce` (using native debounce now)
- `axios` (switched to fetch)

### Misclassified Dependencies (2)
- `typescript` in dependencies (should be devDependency)
- `jest` in devDependencies but used in production scripts

## Commented Code (8 blocks)

- src/legacy/auth.js:120-145 (25 lines commented)
- src/components/Modal.jsx:67-82 (15 lines commented)

## Recommendations

1. **Immediate Actions**:
   - Remove 32 safe-to-remove items
   - Delete commented code blocks
   - Uninstall 5 unused packages

2. **Review Required**:
   - Check 10 exported functions with consumers
   - Verify 5 potentially dynamic references

3. **Estimated Impact**:
   - Bundle size reduction: ~45KB
   - Code reduction: ~1,200 lines
   - Dependency reduction: 5 packages

Detection Strategies

Static Analysis

• Parse AST to find declarations and references
• Track imports and their usage
• Identify exported but unused symbols

Coverage-Based

• Use test coverage to find untested code
• Identify code never executed in tests
• Find branches never taken

Type-Based (TypeScript)

• Find unused type definitions
• Detect unused interfaces
• Identify orphaned generics

Edge Cases to Consider

Dynamic References

// Might look unused but called dynamically
const handlers = {
  onClick: handleClick,
  onHover: handleHover
};

// Called via string
window['initApp']();

Test Code

// Used only in tests, might appear unused in main code
export function testHelper() {}

Public API

// Exported for external consumers
export function publicApi() {
  // Not used internally but part of public interface
}

Language-Specific Tools

• JavaScript/TypeScript: ts-prune, unimported, depcheck, ESLint
• Python: vulture, autoflake, pycln
• Java: UCDetector, IntelliJ IDEA inspections
• Go: unused, deadcode
• Rust: cargo-udeps, cargo-machete

Best Practices

• Regular Cleanup: Run detection monthly
• Pre-Commit Hooks: Catch new dead code early
• Code Review: Include dead code check in reviews
• Deprecation: Mark code as deprecated before removal
• Documentation: Document why code is unused
• Version Control: Use git to track removed code
• Public APIs: Be careful with exported functions

Removal Strategy

1. Start Safe: Remove obvious unused code first
2. Test After Each: Run tests after each removal
3. Check Imports: Update import statements
4. Search Codebase: Grep for string references
5. Review Exports: Consider semver for public packages
6. Document: Note why code was removed in commit

Notes

• Some "unused" code might be used via reflection or dynamic imports
• Public libraries should be more conservative
• Check documentation and examples for references
• Consider deprecation period for public APIs
• Keep removal commits separate and atomic