dead-code-detector

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

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

### Unused Variables
```javascript
// Unused
const result = calculate();
const unused = 42; // Never referenced

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

### Unreachable Code
```javascript
function example() {
  return true;
  console.log('Never executes'); // Dead code
}

if (false) {
  // Dead code block
}
```

### Unused Functions
```javascript
// 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

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

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

### Test Code
```javascript
// Used only in tests, might appear unused in main code
export function testHelper() {}
```

### Public API
```javascript
// 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