AST-Aware Code Refactoring

Overview

Transform code patterns systematically using ast-grep, a syntax-aware tool that finds and replaces code based on abstract syntax trees rather than plain text matching.

When to Use

Use this skill when:

• Renaming functions, classes, or variables across the codebase
• Migrating API usage patterns (e.g., console.log → logger.info)
• Modernizing syntax (e.g., var → const, require → import)
• Replacing deprecated function calls with new equivalents
• Restructuring code architecture (moving, splitting, combining modules)
• Performing bulk transformations that require syntax awareness

Do NOT use this skill for:

• Searching/exploring code without transformations (use ast-search instead)
• Simple text replacements in non-code files (use sed/grep)
• Single-file edits with no pattern repetition (use Edit tool directly)
• Non-structural changes (comments, formatting)
• Bug fixes that don't involve pattern transformation

Quick Reference

# Check if ast-grep is installed
which ast-grep

# Find pattern
ast-grep --pattern 'function $FUNC($$$PARAMS) { $$$ }' --lang javascript

# Search and replace
ast-grep run -p 'var $VAR = $VAL' -r 'const $VAR = $VAL' -l javascript

# Interactive mode
ast-grep run -p 'PATTERN' -r 'REPLACEMENT' -l LANG --interactive

Pattern Syntax

Workflow

Phase 1: Discovery & Analysis

1. Understand the Request: Clarify what structure to refactor
2. Find Current Usage: Use ast-grep to discover all instances
3. Assess Impact: Count affected files/locations, identify dependencies

# Example: Find all class definitions
ast-grep --pattern 'class $CLASS { $$$ }' --lang typescript

Phase 2: Planning

1. Design Transformation: Define find pattern and replacement pattern
2. Create Plan: Summarize what/why/impact with before/after examples

Plan format:

## Refactoring Plan: [Title]

**Goal**: [1-2 sentences]
**Impact**: [X files, Y instances]

**Pattern Match**:
```typescript
// Current pattern

Transform To:

// New pattern

Architecture (if applicable):

Current:                    New:
┌─────────┐                ┌─────────┐
│ Module  │ ──────▶        │ Module  │
└─────────┘                └─────────┘

Additional Steps: [Manual changes needed]

### Phase 3: User Feedback

1. Present plan with options if multiple approaches exist
2. Gather feedback using AskUserQuestion for architectural decisions
3. Confirm approach before proceeding

### Phase 4: Execution

1. **Verify Git State**: Ensure clean working tree or user acknowledges changes
2. **Apply Transformations**:
   ```bash
   # Simple pattern replacement
   ast-grep run -p 'console.log($MSG)' -r 'logger.info($MSG)' -l typescript

   # Interactive for careful review
   ast-grep run -p 'PATTERN' -r 'REPLACEMENT' -l LANG --interactive

3. Verify Changes: Run tests, type checkers, linters
4. Document Results: Summarize changes and any remaining manual work

ast-grep Commands

Basic Search

ast-grep --pattern 'useState($$$)' --lang typescript
ast-grep -p 'function $F() { $$$ }' -l javascript -A 3 -B 2

Search & Replace

ast-grep run -p 'var $VAR = $VAL' -r 'const $VAR = $VAL' -l javascript

Rule-Based Refactoring

# rule.yml
id: modernize-hasOwnProperty
language: typescript
rule:
  pattern: $OBJ.hasOwnProperty($PROP)
fix: Object.hasOwn($OBJ, $PROP)
message: Use Object.hasOwn instead

ast-grep scan -r rule.yml --update-all

Supported Languages

JavaScript, TypeScript, Python, Go, Rust, Java, C, C++, C#, Kotlin, Swift, Ruby, PHP, and more.

Common Mistakes

Safety Guidelines

• Never run destructive refactoring without user approval
• Always show transformation plan with examples before executing
• Use --interactive or --dry-run modes for significant changes
• Verify tests pass after refactoring
• For large refactorings (50+ files), suggest incremental approach
• Work with clean git state, commit refactoring separately from logic changes

Troubleshooting

• Pattern not matching: Verify language flag, check AST structure
• Partial matches: May need multiple patterns for variants
• Syntax errors after refactor: Some edge cases need manual intervention
• Tool not found: Install with brew install ast-grep