Dead Code Elimination

Overview

This document explains how to detect dead code in TypeScript projects.

Tool: ts-remove-unused (tsr)

Installation and Execution

# Run directly with npx (recommended)
npx -y tsr [options] [...entrypoints]

# Or, run with old package name (deprecated)
npx -y @line/ts-remove-unused  # -> warns to use tsr

Basic Usage

1. Check help

npx -y tsr --help

2. Check with single entrypoint

npx -y tsr 'src/index\.ts$'

3. Check with multiple entrypoints

npx -y tsr 'src/index\.ts$' 'src/cli/cli\.ts$'

4. Check including test files

npx -y tsr 'src/index\.ts$' 'src/cli/cli\.ts$' 'test/.*\.ts$' 'src/.*_test\.ts$'

Options

• -w, --write: Write changes directly to files
• -r, --recursive: Recursively check until project is clean
• -p, --project <file>: Path to custom tsconfig.json
• --include-d-ts: Include .d.ts files in the check

Real Analysis Example

1. Initial Run

$ npx -y tsr 'src/index\.ts$'

Results:

• 67 unused exports
• 15 unused files

2. Run including CLI

$ npx -y tsr 'src/index\.ts$' 'src/cli/cli\.ts$'

Results:

• Unused files reduced to 14 (excluding those used by CLI)

3. Run including test files

$ npx -y tsr 'src/index\.ts$' 'src/cli/cli\.ts$' 'test/.*\.ts$' 'src/.*_test\.ts$'

Results:

• Unused files reduced to 4 (excluding those used in tests)

Interpreting Analysis Results

Types of Unused Exports

1. Type Definitions (oxc_types.ts)

   • Many AST types are exported but unused
   • Action: Export only actually used types

2. Internal Utility Functions

   • Example: getNodeLabel, getNodeChildren (apted.ts)
   • Action: Remove export as they are internal implementation

3. Helper Functions

   • Example: collectNodes, findNode (ast_traversal.ts)
   • Action: Consider if needed as public API

Types of Unused Files

1. Test-only Files

   • *_test.ts files
   • Action: Include as test entrypoints

2. Duplicate Functionality

   • Example: function_body_comparer.ts (integrated elsewhere)
   • Action: Delete

3. Experimental Code

   • Example: ast_traversal_with_context.ts
   • Action: Delete or move to experimental/

Recommended Workflow

1. First run analysis only

npx -y tsr 'src/index\.ts$' 'src/cli/cli\.ts$'

2. Review results and decide action plan

• Items that can be deleted
• Items to remove export but keep as internal implementation
• Items to keep for future use

3. Clean up incrementally

• First delete obviously unnecessary items
• Then remove export from internal implementations
• Finally organize type definitions

4. Automatic fixes (carefully)

# Take backup before running
git stash
npx -y tsr --write 'src/index\.ts$' 'src/cli/cli\.ts$'
git diff  # Check changes

Notes

1. Dynamic imports: tsr uses static analysis and cannot detect dynamic imports
2. Type-only exports: export type is also detected as unused
3. Re-exports: Be careful with barrel files (index.ts)

Example in This Project

1. Unused code found in diagnostics

   • Unused imports in semantic_normalizer.ts
   • extractSemanticPatterns function (commented out for potential future use)

2. Actions taken

   • Removed unused imports
   • Kept potentially useful code commented out

3. Results

   • Cleaner codebase
   • Expected reduction in build size