Codemod Best Practices

Comprehensive best practices guide for Codemod (JSSG, ast-grep, workflows), designed for AI agents and LLMs. Contains 48 rules across 11 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Writing new codemods with JSSG or ast-grep
• Designing workflow configurations for migrations
• Debugging pattern matching or AST traversal issues
• Reviewing codemod code for performance and safety
• Setting up test fixtures for transform validation

Rule Categories by Priority

| Priority | Category | Impact | Prefix | |----------|----------|--------|--------| | 1 | AST Understanding | CRITICAL | ast- | | 2 | Pattern Efficiency | CRITICAL | pattern- | | 3 | Parsing Strategy | CRITICAL | parse- | | 4 | Node Traversal | HIGH | traverse- | | 5 | Semantic Analysis | HIGH | semantic- | | 6 | Edit Operations | MEDIUM-HIGH | edit- | | 7 | Workflow Design | MEDIUM-HIGH | workflow- | | 8 | Testing Strategy | MEDIUM | test- | | 9 | State Management | MEDIUM | state- | | 10 | Security and Capabilities | LOW-MEDIUM | security- | | 11 | Package Structure | LOW | pkg- |

Quick Reference

1. AST Understanding (CRITICAL)

• ast-explore-before-writing - Use AST Explorer before writing patterns
• ast-understand-named-vs-anonymous - Understand named vs anonymous nodes
• ast-use-kind-for-precision - Use kind constraint for precision
• ast-field-access-for-structure - Use field access for structural queries
• ast-check-null-before-access - Check null before property access

2. Pattern Efficiency (CRITICAL)

• pattern-use-meta-variables - Use meta variables for flexible matching
• pattern-avoid-overly-generic - Avoid overly generic patterns
• pattern-combine-with-rules - Combine patterns with rule operators
• pattern-use-constraints - Use constraints for reusable matching logic
• pattern-use-relational-patterns - Use relational patterns for context
• pattern-ensure-idempotency - Ensure patterns are idempotent

3. Parsing Strategy (CRITICAL)

• parse-select-correct-parser - Select the correct parser for file type
• parse-handle-embedded-languages - Handle embedded languages with parseAsync
• parse-provide-pattern-context - Provide context for ambiguous patterns
• parse-early-return-non-applicable - Early return for non-applicable files

4. Node Traversal (HIGH)

• traverse-use-find-vs-findall - Use find() for single match, findAll() for multiple
• traverse-single-pass-collection - Collect multiple patterns in single traversal
• traverse-use-stopby-for-depth - Use stopBy to control traversal depth
• traverse-use-siblings-efficiently - Use sibling navigation efficiently
• traverse-cache-repeated-lookups - Cache repeated node lookups

5. Semantic Analysis (HIGH)

• semantic-use-file-scope-first - Use file scope semantic analysis first
• semantic-check-null-results - Handle null semantic analysis results
• semantic-verify-file-ownership - Verify file ownership before cross-file edits
• semantic-cache-cross-file-results - Cache semantic analysis results

6. Edit Operations (MEDIUM-HIGH)

• edit-batch-before-commit - Batch edits before committing
• edit-preserve-formatting - Preserve surrounding formatting in edits
• edit-handle-overlapping-ranges - Handle overlapping edit ranges
• edit-use-flatmap-for-conditional - Use flatMap for conditional edits
• edit-add-imports-correctly - Add imports at correct position

7. Workflow Design (MEDIUM-HIGH)

• workflow-order-nodes-by-dependency - Order nodes by dependency
• workflow-use-matrix-for-parallelism - Use matrix strategy for parallelism
• workflow-use-manual-gates - Use manual gates for critical steps
• workflow-validate-before-run - Validate workflows before running
• workflow-use-conditional-steps - Use conditional steps for dynamic workflows

8. Testing Strategy (MEDIUM)

• test-use-fixture-pairs - Use input/expected fixture pairs
• test-cover-edge-cases - Cover edge cases in test fixtures
• test-use-strictness-levels - Choose appropriate test strictness level
• test-update-fixtures-intentionally - Update test fixtures intentionally
• test-run-on-subset-first - Test on file subset before full run

9. State Management (MEDIUM)

• state-use-for-resumability - Use state for resumable migrations
• state-make-transforms-idempotent - Make transforms idempotent for safe reruns
• state-log-progress-for-observability - Log progress for long-running migrations

10. Security and Capabilities (LOW-MEDIUM)

• security-minimize-capabilities - Minimize requested capabilities
• security-validate-external-inputs - Validate external inputs before use
• security-review-before-running-third-party - Review third-party codemods before running

11. Package Structure (LOW)

• pkg-use-semantic-versioning - Use semantic versioning for packages
• pkg-write-descriptive-metadata - Write descriptive package metadata
• pkg-organize-by-convention - Organize package by convention

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Full Compiled Document

For a complete guide with all rules expanded, see AGENTS.md.